Revert "Temporarily move "metrics" package into impl_core/ for release. (#1426)" (#1432)

This reverts commit d3fa0e5c60903f95c184d7c7bafae8fdd12156ad.

30 files changed, 2831 insertions, 2 deletions

diff --git a/api/src/main/java/io/opencensus/common/ToDoubleFunction.java b/api/src/main/java/io/opencensus/common/ToDoubleFunction.java
index 415f8ca9..eac85793 100644
--- a/api/src/main/java/io/opencensus/common/ToDoubleFunction.java
+++ b/api/src/main/java/io/opencensus/common/ToDoubleFunction.java
@@ -17,7 +17,7 @@
 package io.opencensus.common;
 
 /**
- * Represents a function that produces a double-valued result. See {@code
+ * Represents a function that produces a double-valued result. See {@link
  * io.opencensus.metrics.MetricRegistry} for an example of its use.
  *
  * <p>Note: This class is based on the java.util.ToDoubleFunction class added in Java 1.8. We cannot
diff --git a/api/src/main/java/io/opencensus/common/ToLongFunction.java b/api/src/main/java/io/opencensus/common/ToLongFunction.java
index 727a8b44..26a27b18 100644
--- a/api/src/main/java/io/opencensus/common/ToLongFunction.java
+++ b/api/src/main/java/io/opencensus/common/ToLongFunction.java
@@ -17,7 +17,7 @@
 package io.opencensus.common;
 
 /**
- * Represents a function that produces a long-valued result. See {@code
+ * Represents a function that produces a long-valued result. See {@link
  * io.opencensus.metrics.MetricRegistry} for an example of its use.
  *
  * <p>Note: This class is based on the java.util.ToLongFunction class added in Java 1.8. We cannot
diff --git a/api/src/main/java/io/opencensus/metrics/Distribution.java b/api/src/main/java/io/opencensus/metrics/Distribution.java
new file mode 100644
index 00000000..bdea7762
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/Distribution.java
@@ -0,0 +1,280 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.common.Timestamp;
+import io.opencensus.internal.Utils;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * {@link Distribution} contains summary statistics for a population of values. It optionally
+ * contains a histogram representing the distribution of those values across a set of buckets.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+@AutoValue
+@Immutable
+public abstract class Distribution {
+
+  Distribution() {}
+
+  /**
+   * Creates a {@link Distribution}.
+   *
+   * @param mean mean of the population values.
+   * @param count count of the population values.
+   * @param sumOfSquaredDeviations sum of squared deviations of the population values.
+   * @param bucketBoundaries bucket boundaries of a histogram.
+   * @param buckets {@link Bucket}s of a histogram.
+   * @return a {@code Distribution}.
+   * @since 0.16
+   */
+  public static Distribution create(
+      double mean,
+      long count,
+      double sumOfSquaredDeviations,
+      List<Double> bucketBoundaries,
+      List<Bucket> buckets) {
+    Utils.checkArgument(count >= 0, "count should be non-negative.");
+    Utils.checkArgument(
+        sumOfSquaredDeviations >= 0, "sum of squared deviations should be non-negative.");
+    if (count == 0) {
+      Utils.checkArgument(mean == 0, "mean should be 0 if count is 0.");
+      Utils.checkArgument(
+          sumOfSquaredDeviations == 0, "sum of squared deviations should be 0 if count is 0.");
+    }
+    return new AutoValue_Distribution(
+        mean,
+        count,
+        sumOfSquaredDeviations,
+        copyBucketBounds(bucketBoundaries),
+        copyBucketCount(buckets));
+  }
+
+  private static List<Double> copyBucketBounds(List<Double> bucketBoundaries) {
+    Utils.checkNotNull(bucketBoundaries, "bucketBoundaries list should not be null.");
+    List<Double> bucketBoundariesCopy = new ArrayList<Double>(bucketBoundaries); // Deep copy.
+    // Check if sorted.
+    if (bucketBoundariesCopy.size() > 1) {
+      double lower = bucketBoundariesCopy.get(0);
+      for (int i = 1; i < bucketBoundariesCopy.size(); i++) {
+        double next = bucketBoundariesCopy.get(i);
+        Utils.checkArgument(lower < next, "bucket boundaries not sorted.");
+        lower = next;
+      }
+    }
+    return Collections.unmodifiableList(bucketBoundariesCopy);
+  }
+
+  private static List<Bucket> copyBucketCount(List<Bucket> buckets) {
+    Utils.checkNotNull(buckets, "bucket list should not be null.");
+    List<Bucket> bucketsCopy = new ArrayList<Bucket>(buckets);
+    for (Bucket bucket : bucketsCopy) {
+      Utils.checkNotNull(bucket, "bucket should not be null.");
+    }
+    return Collections.unmodifiableList(bucketsCopy);
+  }
+
+  /**
+   * Returns the aggregated mean.
+   *
+   * @return the aggregated mean.
+   * @since 0.16
+   */
+  public abstract double getMean();
+
+  /**
+   * Returns the aggregated count.
+   *
+   * @return the aggregated count.
+   * @since 0.16
+   */
+  public abstract long getCount();
+
+  /**
+   * Returns the aggregated sum of squared deviations.
+   *
+   * <p>The sum of squared deviations from the mean of the values in the population. For values x_i
+   * this is:
+   *
+   * <p>Sum[i=1..n]((x_i - mean)^2)
+   *
+   * <p>If count is zero then this field must be zero.
+   *
+   * @return the aggregated sum of squared deviations.
+   * @since 0.16
+   */
+  public abstract double getSumOfSquaredDeviations();
+
+  /**
+   * Returns the bucket boundaries of this distribution.
+   *
+   * <p>The bucket boundaries for that histogram are described by bucket_bounds. This defines
+   * size(bucket_bounds) + 1 (= N) buckets. The boundaries for bucket index i are:
+   *
+   * <ul>
+   *   <li>{@code (-infinity, bucket_bounds[i]) for i == 0}
+   *   <li>{@code [bucket_bounds[i-1], bucket_bounds[i]) for 0 < i < N-2}
+   *   <li>{@code [bucket_bounds[i-1], +infinity) for i == N-1}
+   * </ul>
+   *
+   * <p>i.e. an underflow bucket (number 0), zero or more finite buckets (1 through N - 2, and an
+   * overflow bucket (N - 1), with inclusive lower bounds and exclusive upper bounds.
+   *
+   * <p>If bucket_bounds has no elements (zero size), then there is no histogram associated with the
+   * Distribution. If bucket_bounds has only one element, there are no finite buckets, and that
+   * single element is the common boundary of the overflow and underflow buckets. The values must be
+   * monotonically increasing.
+   *
+   * @return the bucket boundaries of this distribution.
+   * @since 0.16
+   */
+  public abstract List<Double> getBucketBoundaries();
+
+  /**
+   * Returns the aggregated histogram {@link Bucket}s.
+   *
+   * @return the aggregated histogram buckets.
+   * @since 0.16
+   */
+  public abstract List<Bucket> getBuckets();
+
+  /**
+   * The histogram bucket of the population values.
+   *
+   * @since 0.16
+   */
+  @AutoValue
+  @Immutable
+  public abstract static class Bucket {
+
+    Bucket() {}
+
+    /**
+     * Creates a {@link Bucket}.
+     *
+     * @param count the number of values in each bucket of the histogram.
+     * @return a {@code Bucket}.
+     * @since 0.16
+     */
+    public static Bucket create(long count) {
+      Utils.checkArgument(count >= 0, "bucket count should be non-negative.");
+      return new AutoValue_Distribution_Bucket(count, null);
+    }
+
+    /**
+     * Creates a {@link Bucket} with an {@link Exemplar}.
+     *
+     * @param count the number of values in each bucket of the histogram.
+     * @param exemplar the {@code Exemplar} of this {@code Bucket}.
+     * @return a {@code Bucket}.
+     * @since 0.16
+     */
+    public static Bucket create(long count, Exemplar exemplar) {
+      Utils.checkArgument(count >= 0, "bucket count should be non-negative.");
+      Utils.checkNotNull(exemplar, "exemplar");
+      return new AutoValue_Distribution_Bucket(count, exemplar);
+    }
+
+    /**
+     * Returns the number of values in each bucket of the histogram.
+     *
+     * @return the number of values in each bucket of the histogram.
+     * @since 0.16
+     */
+    public abstract long getCount();
+
+    /**
+     * Returns the {@link Exemplar} associated with the {@link Bucket}, or {@code null} if there
+     * isn't one.
+     *
+     * @return the {@code Exemplar} associated with the {@code Bucket}, or {@code null} if there
+     *     isn't one.
+     * @since 0.16
+     */
+    @Nullable
+    public abstract Exemplar getExemplar();
+  }
+
+  /**
+   * An example point that may be used to annotate aggregated distribution values, associated with a
+   * histogram bucket.
+   *
+   * @since 0.16
+   */
+  @Immutable
+  @AutoValue
+  public abstract static class Exemplar {
+
+    Exemplar() {}
+
+    /**
+     * Returns value of the {@link Exemplar} point.
+     *
+     * @return value of the {@code Exemplar} point.
+     * @since 0.16
+     */
+    public abstract double getValue();
+
+    /**
+     * Returns the time that this {@link Exemplar}'s value was recorded.
+     *
+     * @return the time that this {@code Exemplar}'s value was recorded.
+     * @since 0.16
+     */
+    public abstract Timestamp getTimestamp();
+
+    /**
+     * Returns the contextual information about the example value, represented as a string map.
+     *
+     * @return the contextual information about the example value.
+     * @since 0.16
+     */
+    public abstract Map<String, String> getAttachments();
+
+    /**
+     * Creates an {@link Exemplar}.
+     *
+     * @param value value of the {@link Exemplar} point.
+     * @param timestamp the time that this {@code Exemplar}'s value was recorded.
+     * @param attachments the contextual information about the example value.
+     * @return an {@code Exemplar}.
+     * @since 0.16
+     */
+    public static Exemplar create(
+        double value, Timestamp timestamp, Map<String, String> attachments) {
+      Utils.checkNotNull(attachments, "attachments");
+      Map<String, String> attachmentsCopy =
+          Collections.unmodifiableMap(new HashMap<String, String>(attachments));
+      for (Entry<String, String> entry : attachmentsCopy.entrySet()) {
+        Utils.checkNotNull(entry.getKey(), "key of attachments");
+        Utils.checkNotNull(entry.getValue(), "value of attachments");
+      }
+      return new AutoValue_Distribution_Exemplar(value, timestamp, attachmentsCopy);
+    }
+  }
+}
diff --git a/api/src/main/java/io/opencensus/metrics/LabelKey.java b/api/src/main/java/io/opencensus/metrics/LabelKey.java
new file mode 100644
index 00000000..01ef0b55
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/LabelKey.java
@@ -0,0 +1,62 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.common.ExperimentalApi;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * The key of a {@code Label} associated with a {@code MetricDescriptor}.
+ *
+ * @since 0.15
+ */
+@ExperimentalApi
+@Immutable
+@AutoValue
+public abstract class LabelKey {
+
+  LabelKey() {}
+
+  /**
+   * Creates a {@link LabelKey}.
+   *
+   * @param key the key of a {@code Label}.
+   * @param description a human-readable description of what this label key represents.
+   * @return a {@code LabelKey}.
+   * @since 0.15
+   */
+  public static LabelKey create(String key, String description) {
+    return new AutoValue_LabelKey(key, description);
+  }
+
+  /**
+   * Returns the key of this {@link LabelKey}.
+   *
+   * @return the key.
+   * @since 0.15
+   */
+  public abstract String getKey();
+
+  /**
+   * Returns the description of this {@link LabelKey}.
+   *
+   * @return the description.
+   * @since 0.15
+   */
+  public abstract String getDescription();
+}
diff --git a/api/src/main/java/io/opencensus/metrics/LabelValue.java b/api/src/main/java/io/opencensus/metrics/LabelValue.java
new file mode 100644
index 00000000..e81b63d2
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/LabelValue.java
@@ -0,0 +1,57 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.common.ExperimentalApi;
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * The value of a {@code Label} associated with a {@code TimeSeries}.
+ *
+ * @since 0.15
+ */
+@ExperimentalApi
+@Immutable
+@AutoValue
+public abstract class LabelValue {
+
+  LabelValue() {}
+
+  /**
+   * Creates a {@link LabelValue}.
+   *
+   * @param value the value of a {@code Label}. {@code null} value indicates an unset {@code
+   *     LabelValue}.
+   * @return a {@code LabelValue}.
+   * @since 0.15
+   */
+  public static LabelValue create(@Nullable String value) {
+    return new AutoValue_LabelValue(value);
+  }
+
+  /**
+   * Returns the value of this {@link LabelValue}. Returns {@code null} if the value is unset and
+   * supposed to be ignored.
+   *
+   * @return the value.
+   * @since 0.15
+   */
+  @Nullable
+  public abstract String getValue();
+}
diff --git a/api/src/main/java/io/opencensus/metrics/Metric.java b/api/src/main/java/io/opencensus/metrics/Metric.java
new file mode 100644
index 00000000..fa0018b3
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/Metric.java
@@ -0,0 +1,98 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.internal.Utils;
+import io.opencensus.metrics.Value.ValueDistribution;
+import io.opencensus.metrics.Value.ValueDouble;
+import io.opencensus.metrics.Value.ValueLong;
+import java.util.List;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * A {@link Metric} with one or more {@link TimeSeries}.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+@Immutable
+@AutoValue
+public abstract class Metric {
+
+  Metric() {}
+
+  /**
+   * Creates a {@link Metric}.
+   *
+   * @param metricDescriptor the {@link MetricDescriptor}.
+   * @param timeSeriesList the {@link TimeSeries} list for this metric.
+   * @return a {@code Metric}.
+   * @since 0.16
+   */
+  public static Metric create(MetricDescriptor metricDescriptor, List<TimeSeries> timeSeriesList) {
+    checkTypeMatch(metricDescriptor.getType(), timeSeriesList);
+    return new AutoValue_Metric(metricDescriptor, timeSeriesList);
+  }
+
+  /**
+   * Returns the {@link MetricDescriptor} of this metric.
+   *
+   * @return the {@code MetricDescriptor} of this metric.
+   * @since 0.16
+   */
+  public abstract MetricDescriptor getMetricDescriptor();
+
+  /**
+   * Returns the {@link TimeSeries} list for this metric.
+   *
+   * <p>The type of the {@link TimeSeries#getPoints()} must match {@link MetricDescriptor.Type}.
+   *
+   * @return the {@code TimeSeriesList} for this metric.
+   * @since 0.16
+   */
+  public abstract List<TimeSeries> getTimeSeriesList();
+
+  private static void checkTypeMatch(MetricDescriptor.Type type, List<TimeSeries> timeSeriesList) {
+    for (TimeSeries timeSeries : timeSeriesList) {
+      for (Point point : timeSeries.getPoints()) {
+        Value value = point.getValue();
+        String valueClassName = "";
+        if (value.getClass().getSuperclass() != null) { // work around nullness check
+          // AutoValue classes should always have a super class.
+          valueClassName = value.getClass().getSuperclass().getSimpleName();
+        }
+        switch (type) {
+          case GAUGE_INT64:
+          case CUMULATIVE_INT64:
+            Utils.checkArgument(
+                value instanceof ValueLong, "Type mismatch: %s, %s.", type, valueClassName);
+            break;
+          case CUMULATIVE_DOUBLE:
+          case GAUGE_DOUBLE:
+            Utils.checkArgument(
+                value instanceof ValueDouble, "Type mismatch: %s, %s.", type, valueClassName);
+            break;
+          case CUMULATIVE_DISTRIBUTION:
+            Utils.checkArgument(
+                value instanceof ValueDistribution, "Type mismatch: %s, %s.", type, valueClassName);
+        }
+      }
+    }
+  }
+}
diff --git a/api/src/main/java/io/opencensus/metrics/MetricDescriptor.java b/api/src/main/java/io/opencensus/metrics/MetricDescriptor.java
new file mode 100644
index 00000000..4558446a
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/MetricDescriptor.java
@@ -0,0 +1,150 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.internal.Utils;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * {@link MetricDescriptor} defines a {@code Metric} type and its schema.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+@Immutable
+@AutoValue
+public abstract class MetricDescriptor {
+
+  MetricDescriptor() {}
+
+  /**
+   * Creates a {@link MetricDescriptor}.
+   *
+   * @param name name of {@code MetricDescriptor}.
+   * @param description description of {@code MetricDescriptor}.
+   * @param unit the metric unit.
+   * @param type type of {@code MetricDescriptor}.
+   * @param labelKeys the label keys associated with the {@code MetricDescriptor}.
+   * @return a {@code MetricDescriptor}.
+   * @since 0.16
+   */
+  public static MetricDescriptor create(
+      String name, String description, String unit, Type type, List<LabelKey> labelKeys) {
+    Utils.checkNotNull(labelKeys, "labelKeys");
+    Utils.checkListElementNotNull(labelKeys, "labelKey");
+    return new AutoValue_MetricDescriptor(
+        name,
+        description,
+        unit,
+        type,
+        Collections.unmodifiableList(new ArrayList<LabelKey>(labelKeys)));
+  }
+
+  /**
+   * Returns the metric descriptor name.
+   *
+   * @return the metric descriptor name.
+   * @since 0.16
+   */
+  public abstract String getName();
+
+  /**
+   * Returns the description of this metric descriptor.
+   *
+   * @return the description of this metric descriptor.
+   * @since 0.16
+   */
+  public abstract String getDescription();
+
+  /**
+   * Returns the unit of this metric descriptor.
+   *
+   * @return the unit of this metric descriptor.
+   * @since 0.16
+   */
+  public abstract String getUnit();
+
+  /**
+   * Returns the type of this metric descriptor.
+   *
+   * @return the type of this metric descriptor.
+   * @since 0.16
+   */
+  public abstract Type getType();
+
+  /**
+   * Returns the label keys associated with this metric descriptor.
+   *
+   * @return the label keys associated with this metric descriptor.
+   * @since 0.16
+   */
+  public abstract List<LabelKey> getLabelKeys();
+
+  /**
+   * The kind of metric. It describes how the data is reported.
+   *
+   * <p>A gauge is an instantaneous measurement of a value.
+   *
+   * <p>A cumulative measurement is a value accumulated over a time interval. In a time series,
+   * cumulative measurements should have the same start time and increasing end times, until an
+   * event resets the cumulative value to zero and sets a new start time for the following points.
+   *
+   * @since 0.16
+   */
+  public enum Type {
+
+    /**
+     * An instantaneous measurement of an int64 value.
+     *
+     * @since 0.16
+     */
+    GAUGE_INT64,
+
+    /**
+     * An instantaneous measurement of a double value.
+     *
+     * @since 0.16
+     */
+    GAUGE_DOUBLE,
+
+    /**
+     * An cumulative measurement of an int64 value.
+     *
+     * @since 0.16
+     */
+    CUMULATIVE_INT64,
+
+    /**
+     * An cumulative measurement of a double value.
+     *
+     * @since 0.16
+     */
+    CUMULATIVE_DOUBLE,
+
+    /**
+     * An cumulative measurement of a distribution value.
+     *
+     * @since 0.16
+     */
+    CUMULATIVE_DISTRIBUTION,
+  }
+}
diff --git a/api/src/main/java/io/opencensus/metrics/MetricProducer.java b/api/src/main/java/io/opencensus/metrics/MetricProducer.java
new file mode 100644
index 00000000..38b5d571
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/MetricProducer.java
@@ -0,0 +1,38 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.metrics.export.MetricProducerManager;
+import java.util.Collection;
+
+/**
+ * A {@link io.opencensus.metrics.Metric} producer that can be registered for exporting using {@link
+ * MetricProducerManager}.
+ *
+ * <p>All implementation MUST be thread-safe.
+ */
+@ExperimentalApi
+public abstract class MetricProducer {
+
+  /**
+   * Returns a collection of produced {@link io.opencensus.metrics.Metric}s to be exported.
+   *
+   * @return a collection of produced {@link io.opencensus.metrics.Metric}s to be exported.
+   */
+  public abstract Collection<Metric> getMetrics();
+}
diff --git a/api/src/main/java/io/opencensus/metrics/MetricRegistry.java b/api/src/main/java/io/opencensus/metrics/MetricRegistry.java
new file mode 100644
index 00000000..ae4d90b0
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/MetricRegistry.java
@@ -0,0 +1,117 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.common.ToDoubleFunction;
+import io.opencensus.common.ToLongFunction;
+import io.opencensus.internal.Utils;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+
+/**
+ * Creates and manages your application's set of metrics. Exporters use the metric registry to
+ * iterate over the set of metrics instrumenting your application, and then further export each
+ * metric to the backend of choice.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+public abstract class MetricRegistry extends MetricProducer {
+  /**
+   * Build a new long gauge to be added to the registry.
+   *
+   * <p>Must be called only once.
+   *
+   * @param name the name of the metric.
+   * @param description the description of the metric.
+   * @param unit the unit of the metric.
+   * @param obj the function argument.
+   * @param function the function to be called.
+   * @since 0.16 @ExperimentalApi
+   */
+  public abstract <T> void addLongGauge(
+      String name,
+      String description,
+      String unit,
+      LinkedHashMap<LabelKey, LabelValue> labels,
+      T obj,
+      ToLongFunction<T> function);
+
+  /**
+   * Build a new double gauge to be added to the registry.
+   *
+   * <p>Must be called only once.
+   *
+   * @param name the name of the metric.
+   * @param description the description of the metric.
+   * @param unit the unit of the metric.
+   * @param obj the function argument.
+   * @param function the function to be called.
+   * @since 0.16 @ExperimentalApi
+   */
+  public abstract <T> void addDoubleGauge(
+      String name,
+      String description,
+      String unit,
+      LinkedHashMap<LabelKey, LabelValue> labels,
+      T obj,
+      ToDoubleFunction<T> function);
+
+  static MetricRegistry newNoopMetricRegistry() {
+    return new NoopMetricRegistry();
+  }
+
+  private static final class NoopMetricRegistry extends MetricRegistry {
+
+    @Override
+    public <T> void addLongGauge(
+        String name,
+        String description,
+        String unit,
+        LinkedHashMap<LabelKey, LabelValue> labels,
+        T obj,
+        ToLongFunction<T> function) {
+      Utils.checkNotNull(name, "name");
+      Utils.checkNotNull(description, "description");
+      Utils.checkNotNull(unit, "unit");
+      Utils.checkNotNull(labels, "labels");
+      Utils.checkNotNull(function, "function");
+    }
+
+    @Override
+    public <T> void addDoubleGauge(
+        String name,
+        String description,
+        String unit,
+        LinkedHashMap<LabelKey, LabelValue> labels,
+        T obj,
+        ToDoubleFunction<T> function) {
+      Utils.checkNotNull(name, "name");
+      Utils.checkNotNull(description, "description");
+      Utils.checkNotNull(unit, "unit");
+      Utils.checkNotNull(labels, "labels");
+      Utils.checkNotNull(function, "function");
+    }
+
+    @Override
+    public Collection<Metric> getMetrics() {
+      return Collections.emptyList();
+    }
+  }
+}
diff --git a/api/src/main/java/io/opencensus/metrics/Metrics.java b/api/src/main/java/io/opencensus/metrics/Metrics.java
new file mode 100644
index 00000000..521fbd08
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/Metrics.java
@@ -0,0 +1,96 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.internal.DefaultVisibilityForTesting;
+import io.opencensus.internal.Provider;
+import io.opencensus.metrics.export.ExportComponent;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+import javax.annotation.Nullable;
+
+/**
+ * Class for accessing the default {@link MetricsComponent}.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+public final class Metrics {
+  private static final Logger logger = Logger.getLogger(Metrics.class.getName());
+  private static final MetricsComponent metricsComponent =
+      loadMetricsComponent(MetricsComponent.class.getClassLoader());
+
+  /**
+   * Returns the global {@link ExportComponent}.
+   *
+   * @return the global {@code ExportComponent}.
+   * @since 0.16
+   */
+  public static ExportComponent getExportComponent() {
+    return metricsComponent.getExportComponent();
+  }
+
+  /**
+   * Returns the global {@link MetricRegistry}.
+   *
+   * <p>This {@code MetricRegistry} is already added to the global {@link
+   * io.opencensus.metrics.export.MetricProducerManager}.
+   *
+   * @return the global {@code MetricRegistry}.
+   * @since 0.16
+   */
+  public static MetricRegistry getMetricRegistry() {
+    return metricsComponent.getMetricRegistry();
+  }
+
+  // Any provider that may be used for MetricsComponent can be added here.
+  @DefaultVisibilityForTesting
+  static MetricsComponent loadMetricsComponent(@Nullable ClassLoader classLoader) {
+    try {
+      // Call Class.forName with literal string name of the class to help shading tools.
+      return Provider.createInstance(
+          Class.forName(
+              "io.opencensus.impl.metrics.MetricsComponentImpl", /*initialize=*/ true, classLoader),
+          MetricsComponent.class);
+    } catch (ClassNotFoundException e) {
+      logger.log(
+          Level.FINE,
+          "Couldn't load full implementation for MetricsComponent, now trying to load lite "
+              + "implementation.",
+          e);
+    }
+    try {
+      // Call Class.forName with literal string name of the class to help shading tools.
+      return Provider.createInstance(
+          Class.forName(
+              "io.opencensus.impllite.metrics.MetricsComponentImplLite",
+              /*initialize=*/ true,
+              classLoader),
+          MetricsComponent.class);
+    } catch (ClassNotFoundException e) {
+      logger.log(
+          Level.FINE,
+          "Couldn't load lite implementation for MetricsComponent, now using default "
+              + "implementation for MetricsComponent.",
+          e);
+    }
+    return MetricsComponent.newNoopMetricsComponent();
+  }
+
+  private Metrics() {}
+}
diff --git a/api/src/main/java/io/opencensus/metrics/MetricsComponent.java b/api/src/main/java/io/opencensus/metrics/MetricsComponent.java
new file mode 100644
index 00000000..08d954ef
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/MetricsComponent.java
@@ -0,0 +1,71 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.metrics.export.ExportComponent;
+
+/**
+ * Class that holds the implementation instance for {@link ExportComponent}.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+public abstract class MetricsComponent {
+
+  /**
+   * Returns the {@link ExportComponent} with the provided implementation. If no implementation is
+   * provided then no-op implementations will be used.
+   *
+   * @return the {@link ExportComponent} implementation.
+   * @since 0.16
+   */
+  public abstract ExportComponent getExportComponent();
+
+  /**
+   * Returns the {@link MetricRegistry} with the provided implementation.
+   *
+   * @return the {@link MetricRegistry} implementation.
+   * @since 0.16
+   */
+  public abstract MetricRegistry getMetricRegistry();
+
+  /**
+   * Returns an instance that contains no-op implementations for all the instances.
+   *
+   * @return an instance that contains no-op implementations for all the instances.
+   */
+  static MetricsComponent newNoopMetricsComponent() {
+    return new NoopMetricsComponent();
+  }
+
+  private static final class NoopMetricsComponent extends MetricsComponent {
+    private static final ExportComponent EXPORT_COMPONENT =
+        ExportComponent.newNoopExportComponent();
+    private static final MetricRegistry METRIC_REGISTRY = MetricRegistry.newNoopMetricRegistry();
+
+    @Override
+    public ExportComponent getExportComponent() {
+      return EXPORT_COMPONENT;
+    }
+
+    @Override
+    public MetricRegistry getMetricRegistry() {
+      return METRIC_REGISTRY;
+    }
+  }
+}
diff --git a/api/src/main/java/io/opencensus/metrics/Point.java b/api/src/main/java/io/opencensus/metrics/Point.java
new file mode 100644
index 00000000..e66b273d
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/Point.java
@@ -0,0 +1,63 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.common.Timestamp;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * A timestamped measurement of a {@code TimeSeries}.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+@AutoValue
+@Immutable
+public abstract class Point {
+
+  Point() {}
+
+  /**
+   * Creates a {@link Point}.
+   *
+   * @param value the {@link Value} of this {@link Point}.
+   * @param timestamp the {@link Timestamp} when this {@link Point} was recorded.
+   * @return a {@code Point}.
+   * @since 0.16
+   */
+  public static Point create(Value value, Timestamp timestamp) {
+    return new AutoValue_Point(value, timestamp);
+  }
+
+  /**
+   * Returns the {@link Value}.
+   *
+   * @return the {@code Value}.
+   * @since 0.16
+   */
+  public abstract Value getValue();
+
+  /**
+   * Returns the {@link Timestamp} when this {@link Point} was recorded.
+   *
+   * @return the {@code Timestamp}.
+   * @since 0.16
+   */
+  public abstract Timestamp getTimestamp();
+}
diff --git a/api/src/main/java/io/opencensus/metrics/TimeSeries.java b/api/src/main/java/io/opencensus/metrics/TimeSeries.java
new file mode 100644
index 00000000..c62b6b72
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/TimeSeries.java
@@ -0,0 +1,94 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.common.Timestamp;
+import io.opencensus.internal.Utils;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * A collection of data points that describes the time-varying values of a {@code Metric}.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+@Immutable
+@AutoValue
+public abstract class TimeSeries {
+
+  TimeSeries() {}
+
+  /**
+   * Creates a {@link TimeSeries}.
+   *
+   * @param labelValues the {@code LabelValue}s that uniquely identify this {@code TimeSeries}.
+   * @param points the data {@code Point}s of this {@code TimeSeries}.
+   * @param startTimestamp the start {@code Timestamp} of this {@code TimeSeries}. Must be non-null
+   *     for cumulative {@code Point}s.
+   * @return a {@code TimeSeries}.
+   * @since 0.16
+   */
+  public static TimeSeries create(
+      List<LabelValue> labelValues, List<Point> points, @Nullable Timestamp startTimestamp) {
+    // Fail fast on null lists to prevent NullPointerException when copying the lists.
+    Utils.checkNotNull(labelValues, "labelValues");
+    Utils.checkNotNull(points, "points");
+    Utils.checkListElementNotNull(labelValues, "labelValue");
+    Utils.checkListElementNotNull(points, "point");
+    return new AutoValue_TimeSeries(
+        Collections.unmodifiableList(new ArrayList<LabelValue>(labelValues)),
+        Collections.unmodifiableList(new ArrayList<Point>(points)),
+        startTimestamp);
+  }
+
+  /**
+   * Returns the set of {@link LabelValue}s that uniquely identify this {@link TimeSeries}.
+   *
+   * <p>Apply to all {@link Point}s.
+   *
+   * <p>The order of {@link LabelValue}s must match that of {@link LabelKey}s in the {@code
+   * MetricDescriptor}.
+   *
+   * @return the {@code LabelValue}s.
+   * @since 0.16
+   */
+  public abstract List<LabelValue> getLabelValues();
+
+  /**
+   * Returns the data {@link Point}s of this {@link TimeSeries}.
+   *
+   * @return the data {@code Point}s.
+   * @since 0.16
+   */
+  public abstract List<Point> getPoints();
+
+  /**
+   * Returns the start {@link Timestamp} of this {@link TimeSeries} if the {@link Point}s are
+   * cumulative, or {@code null} if the {@link Point}s are gauge.
+   *
+   * @return the start {@code Timestamp} or {@code null}.
+   * @since 0.16
+   */
+  @Nullable
+  public abstract Timestamp getStartTimestamp();
+}
diff --git a/api/src/main/java/io/opencensus/metrics/Value.java b/api/src/main/java/io/opencensus/metrics/Value.java
new file mode 100644
index 00000000..c8fe48f8
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/Value.java
@@ -0,0 +1,196 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.common.Function;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * The actual point value for a {@link Point}.
+ *
+ * <p>Currently there are three types of {@link Value}:
+ *
+ * <ul>
+ *   <li>{@code double}
+ *   <li>{@code long}
+ *   <li>{@link Distribution}
+ * </ul>
+ *
+ * <p>Each {@link Point} contains exactly one of the three {@link Value} types.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+@Immutable
+public abstract class Value {
+
+  Value() {}
+
+  /**
+   * Returns a double {@link Value}.
+   *
+   * @param value value in double.
+   * @return a double {@code Value}.
+   * @since 0.16
+   */
+  public static Value doubleValue(double value) {
+    return ValueDouble.create(value);
+  }
+
+  /**
+   * Returns a long {@link Value}.
+   *
+   * @param value value in long.
+   * @return a long {@code Value}.
+   * @since 0.16
+   */
+  public static Value longValue(long value) {
+    return ValueLong.create(value);
+  }
+
+  /**
+   * Returns a {@link Distribution} {@link Value}.
+   *
+   * @param value value in {@link Distribution}.
+   * @return a {@code Distribution} {@code Value}.
+   * @since 0.16
+   */
+  public static Value distributionValue(Distribution value) {
+    return ValueDistribution.create(value);
+  }
+
+  /**
+   * Applies the given match function to the underlying data type.
+   *
+   * @since 0.16
+   */
+  public abstract <T> T match(
+      Function<? super Double, T> doubleFunction,
+      Function<? super Long, T> longFunction,
+      Function<? super Distribution, T> distributionFunction,
+      Function<? super Value, T> defaultFunction);
+
+  /** A 64-bit double-precision floating-point {@link Value}. */
+  @AutoValue
+  @Immutable
+  abstract static class ValueDouble extends Value {
+
+    ValueDouble() {}
+
+    @Override
+    public final <T> T match(
+        Function<? super Double, T> doubleFunction,
+        Function<? super Long, T> longFunction,
+        Function<? super Distribution, T> distributionFunction,
+        Function<? super Value, T> defaultFunction) {
+      return doubleFunction.apply(getValue());
+    }
+
+    /**
+     * Creates a {@link ValueDouble}.
+     *
+     * @param value the value in double.
+     * @return a {@code ValueDouble}.
+     */
+    static ValueDouble create(double value) {
+      return new AutoValue_Value_ValueDouble(value);
+    }
+
+    /**
+     * Returns the double value.
+     *
+     * @return the double value.
+     */
+    abstract double getValue();
+  }
+
+  /** A 64-bit integer {@link Value}. */
+  @AutoValue
+  @Immutable
+  abstract static class ValueLong extends Value {
+
+    ValueLong() {}
+
+    @Override
+    public final <T> T match(
+        Function<? super Double, T> doubleFunction,
+        Function<? super Long, T> longFunction,
+        Function<? super Distribution, T> distributionFunction,
+        Function<? super Value, T> defaultFunction) {
+      return longFunction.apply(getValue());
+    }
+
+    /**
+     * Creates a {@link ValueLong}.
+     *
+     * @param value the value in long.
+     * @return a {@code ValueLong}.
+     */
+    static ValueLong create(long value) {
+      return new AutoValue_Value_ValueLong(value);
+    }
+
+    /**
+     * Returns the long value.
+     *
+     * @return the long value.
+     */
+    abstract long getValue();
+  }
+
+  /**
+   * {@link ValueDistribution} contains summary statistics for a population of values. It optionally
+   * contains a histogram representing the distribution of those values across a set of buckets.
+   */
+  @AutoValue
+  @Immutable
+  abstract static class ValueDistribution extends Value {
+
+    ValueDistribution() {}
+
+    @Override
+    public final <T> T match(
+        Function<? super Double, T> doubleFunction,
+        Function<? super Long, T> longFunction,
+        Function<? super Distribution, T> distributionFunction,
+        Function<? super Value, T> defaultFunction) {
+      return distributionFunction.apply(getValue());
+    }
+
+    /**
+     * Creates a {@link ValueDistribution}.
+     *
+     * @param value the {@link Distribution} value.
+     * @return a {@code ValueDistribution}.
+     */
+    static ValueDistribution create(Distribution value) {
+      return new AutoValue_Value_ValueDistribution(value);
+    }
+
+    /**
+     * Returns the {@link Distribution} value.
+     *
+     * @return the {@code Distribution} value.
+     */
+    abstract Distribution getValue();
+  }
+
+  // TODO(songya): Add support for Summary type.
+  // This is an aggregation that produces percentiles directly.
+}
diff --git a/api/src/main/java/io/opencensus/metrics/export/ExportComponent.java b/api/src/main/java/io/opencensus/metrics/export/ExportComponent.java
new file mode 100644
index 00000000..bab65d13
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/export/ExportComponent.java
@@ -0,0 +1,60 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics.export;
+
+import io.opencensus.common.ExperimentalApi;
+
+/**
+ * Class that holds the implementation instance for {@link MetricProducerManager}.
+ *
+ * <p>Unless otherwise noted all methods (on component) results are cacheable.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+public abstract class ExportComponent {
+  /**
+   * Returns the no-op implementation of the {@code ExportComponent}.
+   *
+   * @return the no-op implementation of the {@code ExportComponent}.
+   * @since 0.16
+   */
+  public static ExportComponent newNoopExportComponent() {
+    return new NoopExportComponent();
+  }
+
+  /**
+   * Returns the global {@link MetricProducerManager} which can be used to register handlers to
+   * export all the recorded metrics.
+   *
+   * @return the implementation of the {@code MetricExporter} or no-op if no implementation linked
+   *     in the binary.
+   * @since 0.16
+   */
+  public abstract MetricProducerManager getMetricProducerManager();
+
+  private static final class NoopExportComponent extends ExportComponent {
+
+    private static final MetricProducerManager METRIC_PRODUCER_MANAGER =
+        MetricProducerManager.newNoopMetricProducerManager();
+
+    @Override
+    public MetricProducerManager getMetricProducerManager() {
+      return METRIC_PRODUCER_MANAGER;
+    }
+  }
+}
diff --git a/api/src/main/java/io/opencensus/metrics/export/MetricProducerManager.java b/api/src/main/java/io/opencensus/metrics/export/MetricProducerManager.java
new file mode 100644
index 00000000..c5acf174
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/export/MetricProducerManager.java
@@ -0,0 +1,89 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics.export;
+
+import io.opencensus.common.ExperimentalApi;
+import io.opencensus.internal.Utils;
+import io.opencensus.metrics.MetricProducer;
+import java.util.Collections;
+import java.util.Set;
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * Keeps a set of {@link MetricProducer} that is used by exporters to determine the metrics that
+ * need to be exported.
+ *
+ * @since 0.16
+ */
+@ExperimentalApi
+@ThreadSafe
+public abstract class MetricProducerManager {
+
+  /**
+   * Adds the {@link MetricProducer} to the manager if it is not already present.
+   *
+   * @param metricProducer the {@code MetricProducer} to be added to the manager.
+   * @since 0.16
+   */
+  public abstract void add(MetricProducer metricProducer);
+
+  /**
+   * Removes the {@link MetricProducer} to the manager if it is present.
+   *
+   * @param metricProducer the {@code MetricProducer} to be removed from the manager.
+   * @since 0.16
+   */
+  public abstract void remove(MetricProducer metricProducer);
+
+  /**
+   * Returns all registered {@link MetricProducer}s that should be exported.
+   *
+   * <p>This method should be used by any metrics exporter that automatically exports data for
+   * {@code MetricProducer} registered with the {@code MetricProducerManager}.
+   *
+   * @return all registered {@code MetricProducer}s that should be exported.
+   * @since 0.16
+   */
+  public abstract Set<MetricProducer> getAllMetricProducer();
+
+  /**
+   * Returns a no-op implementation for {@link MetricProducerManager}.
+   *
+   * @return a no-op implementation for {@code MetricProducerManager}.
+   */
+  static MetricProducerManager newNoopMetricProducerManager() {
+    return new NoopMetricProducerManager();
+  }
+
+  private static final class NoopMetricProducerManager extends MetricProducerManager {
+
+    @Override
+    public void add(MetricProducer metricProducer) {
+      Utils.checkNotNull(metricProducer, "metricProducer");
+    }
+
+    @Override
+    public void remove(MetricProducer metricProducer) {
+      Utils.checkNotNull(metricProducer, "metricProducer");
+    }
+
+    @Override
+    public Set<MetricProducer> getAllMetricProducer() {
+      return Collections.emptySet();
+    }
+  }
+}
diff --git a/api/src/main/java/io/opencensus/metrics/package-info.java b/api/src/main/java/io/opencensus/metrics/package-info.java
new file mode 100644
index 00000000..33eadf0c
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/package-info.java
@@ -0,0 +1,32 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * This package describes the Metrics data model. Metrics are a data model for what stats exporters
+ * take as input. This data model may eventually become the wire format for metrics.
+ *
+ * <p>WARNING: Currently all the public classes under this package are marked as {@link
+ * io.opencensus.common.ExperimentalApi}. The classes and APIs under {@link io.opencensus.metrics}
+ * are likely to get backwards-incompatible updates in the future. DO NOT USE except for
+ * experimental purposes.
+ *
+ * <p>Please see
+ * https://github.com/census-instrumentation/opencensus-specs/blob/master/stats/Metrics.md and
+ * https://github.com/census-instrumentation/opencensus-proto/blob/master/opencensus/proto/stats/metrics/metrics.proto
+ * for more details.
+ */
+@io.opencensus.common.ExperimentalApi
+package io.opencensus.metrics;
diff --git a/api/src/test/java/io/opencensus/metrics/DistributionTest.java b/api/src/test/java/io/opencensus/metrics/DistributionTest.java
new file mode 100644
index 00000000..d511e317
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/DistributionTest.java
@@ -0,0 +1,226 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.testing.EqualsTester;
+import io.opencensus.common.Timestamp;
+import io.opencensus.metrics.Distribution.Bucket;
+import io.opencensus.metrics.Distribution.Exemplar;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link Value}. */
+@RunWith(JUnit4.class)
+public class DistributionTest {
+
+  @Rule public final ExpectedException thrown = ExpectedException.none();
+
+  private static final Timestamp TIMESTAMP = Timestamp.create(1, 0);
+  private static final Map<String, String> ATTACHMENTS = Collections.singletonMap("key", "value");
+
+  @Test
+  public void createAndGet_Bucket() {
+    Bucket bucket = Bucket.create(98);
+    assertThat(bucket.getCount()).isEqualTo(98);
+    assertThat(bucket.getExemplar()).isNull();
+  }
+
+  @Test
+  public void createAndGet_BucketWithExemplar() {
+    Exemplar exemplar = Exemplar.create(12.2, TIMESTAMP, ATTACHMENTS);
+    Bucket bucket = Bucket.create(7, exemplar);
+    assertThat(bucket.getCount()).isEqualTo(7);
+    assertThat(bucket.getExemplar()).isEqualTo(exemplar);
+  }
+
+  @Test
+  public void createBucket_preventNullExemplar() {
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("exemplar");
+    Bucket.create(1, null);
+  }
+
+  @Test
+  public void createAndGet_Exemplar() {
+    Exemplar exemplar = Exemplar.create(-9.9, TIMESTAMP, ATTACHMENTS);
+    assertThat(exemplar.getValue()).isEqualTo(-9.9);
+    assertThat(exemplar.getTimestamp()).isEqualTo(TIMESTAMP);
+    assertThat(exemplar.getAttachments()).isEqualTo(ATTACHMENTS);
+  }
+
+  @Test
+  public void createAndGet_Distribution() {
+    Exemplar exemplar = Exemplar.create(15.0, TIMESTAMP, ATTACHMENTS);
+    List<Double> bucketBounds = Arrays.asList(-1.0, 0.0, 1.0);
+    List<Bucket> buckets =
+        Arrays.asList(
+            Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4, exemplar));
+    Distribution distribution = Distribution.create(6.6, 10, 678.54, bucketBounds, buckets);
+    assertThat(distribution.getMean()).isEqualTo(6.6);
+    assertThat(distribution.getCount()).isEqualTo(10);
+    assertThat(distribution.getSumOfSquaredDeviations()).isEqualTo(678.54);
+    assertThat(distribution.getBucketBoundaries())
+        .containsExactlyElementsIn(bucketBounds)
+        .inOrder();
+    assertThat(distribution.getBuckets()).containsExactlyElementsIn(buckets).inOrder();
+  }
+
+  @Test
+  public void createBucket_NegativeCount() {
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("bucket count should be non-negative.");
+    Bucket.create(-5);
+  }
+
+  @Test
+  public void createExemplar_PreventNullAttachments() {
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("attachments");
+    Exemplar.create(15, TIMESTAMP, null);
+  }
+
+  @Test
+  public void createExemplar_PreventNullAttachmentKey() {
+    Map<String, String> attachments = Collections.singletonMap(null, "value");
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("key of attachment");
+    Exemplar.create(15, TIMESTAMP, attachments);
+  }
+
+  @Test
+  public void createExemplar_PreventNullAttachmentValue() {
+    Map<String, String> attachments = Collections.singletonMap("key", null);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("value of attachment");
+    Exemplar.create(15, TIMESTAMP, attachments);
+  }
+
+  @Test
+  public void createDistribution_NegativeCount() {
+    List<Double> bucketBounds = Arrays.asList(-1.0, 0.0, 1.0);
+    List<Bucket> buckets =
+        Arrays.asList(Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4));
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("count should be non-negative.");
+    Distribution.create(6.6, -10, 678.54, bucketBounds, buckets);
+  }
+
+  @Test
+  public void createDistribution_NegativeSumOfSquaredDeviations() {
+    List<Double> bucketBounds = Arrays.asList(-1.0, 0.0, 1.0);
+    List<Bucket> buckets =
+        Arrays.asList(Bucket.create(0), Bucket.create(0), Bucket.create(0), Bucket.create(0));
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("sum of squared deviations should be non-negative.");
+    Distribution.create(6.6, 0, -678.54, bucketBounds, buckets);
+  }
+
+  @Test
+  public void createDistribution_ZeroCountAndPositiveMean() {
+    List<Double> bucketBounds = Arrays.asList(-1.0, 0.0, 1.0);
+    List<Bucket> buckets =
+        Arrays.asList(Bucket.create(0), Bucket.create(0), Bucket.create(0), Bucket.create(0));
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("mean should be 0 if count is 0.");
+    Distribution.create(6.6, 0, 0, bucketBounds, buckets);
+  }
+
+  @Test
+  public void createDistribution_ZeroCountAndSumOfSquaredDeviations() {
+    List<Double> bucketBounds = Arrays.asList(-1.0, 0.0, 1.0);
+    List<Bucket> buckets =
+        Arrays.asList(Bucket.create(0), Bucket.create(0), Bucket.create(0), Bucket.create(0));
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("sum of squared deviations should be 0 if count is 0.");
+    Distribution.create(0, 0, 678.54, bucketBounds, buckets);
+  }
+
+  @Test
+  public void createDistribution_NullBucketBounds() {
+    List<Bucket> buckets =
+        Arrays.asList(Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4));
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("bucketBoundaries list should not be null.");
+    Distribution.create(6.6, 10, 678.54, null, buckets);
+  }
+
+  @Test
+  public void createDistribution_UnorderedBucketBounds() {
+    List<Double> bucketBounds = Arrays.asList(0.0, -1.0, 1.0);
+    List<Bucket> buckets =
+        Arrays.asList(Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4));
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("bucket boundaries not sorted.");
+    Distribution.create(6.6, 10, 678.54, bucketBounds, buckets);
+  }
+
+  @Test
+  public void createDistribution_NullBucketList() {
+    List<Double> bucketBounds = Arrays.asList(-1.0, 0.0, 1.0);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("bucket list should not be null.");
+    Distribution.create(6.6, 10, 678.54, bucketBounds, null);
+  }
+
+  @Test
+  public void createDistribution_NullBucket() {
+    List<Double> bucketBounds = Arrays.asList(-1.0, 0.0, 1.0);
+    List<Bucket> buckets =
+        Arrays.asList(Bucket.create(3), Bucket.create(1), null, Bucket.create(4));
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("bucket should not be null.");
+    Distribution.create(6.6, 10, 678.54, bucketBounds, buckets);
+  }
+
+  @Test
+  public void testEquals() {
+    new EqualsTester()
+        .addEqualityGroup(
+            Distribution.create(
+                10,
+                10,
+                1,
+                Arrays.asList(-5.0, 0.0, 5.0),
+                Arrays.asList(
+                    Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4))),
+            Distribution.create(
+                10,
+                10,
+                1,
+                Arrays.asList(-5.0, 0.0, 5.0),
+                Arrays.asList(
+                    Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4))))
+        .addEqualityGroup(
+            Distribution.create(
+                -7,
+                10,
+                23.456,
+                Arrays.asList(-5.0, 0.0, 5.0),
+                Arrays.asList(
+                    Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4))))
+        .testEquals();
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/LabelKeyTest.java b/api/src/test/java/io/opencensus/metrics/LabelKeyTest.java
new file mode 100644
index 00000000..83f2b59a
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/LabelKeyTest.java
@@ -0,0 +1,86 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.testing.EqualsTester;
+import java.util.Arrays;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link LabelKey}. */
+@RunWith(JUnit4.class)
+public class LabelKeyTest {
+
+  private static final LabelKey KEY = LabelKey.create("key", "description");
+
+  @Test
+  public void testGetKey() {
+    assertThat(KEY.getKey()).isEqualTo("key");
+  }
+
+  @Test
+  public void testGetDescription() {
+    assertThat(KEY.getDescription()).isEqualTo("description");
+  }
+
+  @Test
+  public void create_NoLengthConstraint() {
+    // We have a length constraint of 256-characters for TagKey. That constraint doesn't apply to
+    // LabelKey.
+    char[] chars = new char[300];
+    Arrays.fill(chars, 'k');
+    String key = new String(chars);
+    assertThat(LabelKey.create(key, "").getKey()).isEqualTo(key);
+  }
+
+  @Test
+  public void create_WithUnprintableChars() {
+    String key = "\2ab\3cd";
+    String description = "\4ef\5gh";
+    LabelKey labelKey = LabelKey.create(key, description);
+    assertThat(labelKey.getKey()).isEqualTo(key);
+    assertThat(labelKey.getDescription()).isEqualTo(description);
+  }
+
+  @Test
+  public void create_WithNonAsciiChars() {
+    String key = "键";
+    String description = "测试用键";
+    LabelKey nonAsciiKey = LabelKey.create(key, description);
+    assertThat(nonAsciiKey.getKey()).isEqualTo(key);
+    assertThat(nonAsciiKey.getDescription()).isEqualTo(description);
+  }
+
+  @Test
+  public void create_Empty() {
+    LabelKey emptyKey = LabelKey.create("", "");
+    assertThat(emptyKey.getKey()).isEmpty();
+    assertThat(emptyKey.getDescription()).isEmpty();
+  }
+
+  @Test
+  public void testLabelKeyEquals() {
+    new EqualsTester()
+        .addEqualityGroup(LabelKey.create("foo", ""), LabelKey.create("foo", ""))
+        .addEqualityGroup(LabelKey.create("foo", "description"))
+        .addEqualityGroup(LabelKey.create("bar", ""))
+        .testEquals();
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/LabelValueTest.java b/api/src/test/java/io/opencensus/metrics/LabelValueTest.java
new file mode 100644
index 00000000..e5526b2f
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/LabelValueTest.java
@@ -0,0 +1,74 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.testing.EqualsTester;
+import java.util.Arrays;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link LabelValue}. */
+@RunWith(JUnit4.class)
+public class LabelValueTest {
+
+  private static final LabelValue VALUE = LabelValue.create("value");
+  private static final LabelValue UNSET = LabelValue.create(null);
+  private static final LabelValue EMPTY = LabelValue.create("");
+
+  @Test
+  public void testGetValue() {
+    assertThat(VALUE.getValue()).isEqualTo("value");
+    assertThat(UNSET.getValue()).isNull();
+    assertThat(EMPTY.getValue()).isEmpty();
+  }
+
+  @Test
+  public void create_NoLengthConstraint() {
+    // We have a length constraint of 256-characters for TagValue. That constraint doesn't apply to
+    // LabelValue.
+    char[] chars = new char[300];
+    Arrays.fill(chars, 'v');
+    String value = new String(chars);
+    assertThat(LabelValue.create(value).getValue()).isEqualTo(value);
+  }
+
+  @Test
+  public void create_WithUnprintableChars() {
+    String value = "\2ab\3cd";
+    assertThat(LabelValue.create(value).getValue()).isEqualTo(value);
+  }
+
+  @Test
+  public void create_WithNonAsciiChars() {
+    String value = "值";
+    LabelValue nonAsciiValue = LabelValue.create(value);
+    assertThat(nonAsciiValue.getValue()).isEqualTo(value);
+  }
+
+  @Test
+  public void testLabelValueEquals() {
+    new EqualsTester()
+        .addEqualityGroup(LabelValue.create("foo"), LabelValue.create("foo"))
+        .addEqualityGroup(UNSET)
+        .addEqualityGroup(EMPTY)
+        .addEqualityGroup(LabelValue.create("bar"))
+        .testEquals();
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/MetricDescriptorTest.java b/api/src/test/java/io/opencensus/metrics/MetricDescriptorTest.java
new file mode 100644
index 00000000..9c0a42fc
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/MetricDescriptorTest.java
@@ -0,0 +1,102 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.testing.EqualsTester;
+import io.opencensus.metrics.MetricDescriptor.Type;
+import java.util.Arrays;
+import java.util.List;
+import org.hamcrest.CoreMatchers;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link MetricDescriptor}. */
+@RunWith(JUnit4.class)
+public class MetricDescriptorTest {
+
+  @Rule public final ExpectedException thrown = ExpectedException.none();
+
+  private static final String METRIC_NAME_1 = "metric1";
+  private static final String METRIC_NAME_2 = "metric2";
+  private static final String DESCRIPTION = "Metric description.";
+  private static final String UNIT = "kb/s";
+  private static final LabelKey KEY_1 = LabelKey.create("key1", "some key");
+  private static final LabelKey KEY_2 = LabelKey.create("key2", "some other key");
+
+  @Test
+  public void testGet() {
+    MetricDescriptor metricDescriptor =
+        MetricDescriptor.create(
+            METRIC_NAME_1, DESCRIPTION, UNIT, Type.GAUGE_DOUBLE, Arrays.asList(KEY_1, KEY_2));
+    assertThat(metricDescriptor.getName()).isEqualTo(METRIC_NAME_1);
+    assertThat(metricDescriptor.getDescription()).isEqualTo(DESCRIPTION);
+    assertThat(metricDescriptor.getUnit()).isEqualTo(UNIT);
+    assertThat(metricDescriptor.getType()).isEqualTo(Type.GAUGE_DOUBLE);
+    assertThat(metricDescriptor.getLabelKeys()).containsExactly(KEY_1, KEY_2).inOrder();
+  }
+
+  @Test
+  public void preventNullLabelKeyList() {
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage(CoreMatchers.equalTo("labelKeys"));
+    MetricDescriptor.create(METRIC_NAME_1, DESCRIPTION, UNIT, Type.GAUGE_DOUBLE, null);
+  }
+
+  @Test
+  public void preventNullLabelKey() {
+    List<LabelKey> keys = Arrays.asList(KEY_1, null);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage(CoreMatchers.equalTo("labelKey"));
+    MetricDescriptor.create(METRIC_NAME_1, DESCRIPTION, UNIT, Type.GAUGE_DOUBLE, keys);
+  }
+
+  @Test
+  public void testEquals() {
+    new EqualsTester()
+        .addEqualityGroup(
+            MetricDescriptor.create(
+                METRIC_NAME_1, DESCRIPTION, UNIT, Type.GAUGE_DOUBLE, Arrays.asList(KEY_1, KEY_2)),
+            MetricDescriptor.create(
+                METRIC_NAME_1, DESCRIPTION, UNIT, Type.GAUGE_DOUBLE, Arrays.asList(KEY_1, KEY_2)))
+        .addEqualityGroup(
+            MetricDescriptor.create(
+                METRIC_NAME_2, DESCRIPTION, UNIT, Type.GAUGE_DOUBLE, Arrays.asList(KEY_1, KEY_2)))
+        .addEqualityGroup(
+            MetricDescriptor.create(
+                METRIC_NAME_2, DESCRIPTION, UNIT, Type.GAUGE_INT64, Arrays.asList(KEY_1, KEY_2)))
+        .addEqualityGroup(
+            MetricDescriptor.create(
+                METRIC_NAME_1,
+                DESCRIPTION,
+                UNIT,
+                Type.CUMULATIVE_DISTRIBUTION,
+                Arrays.asList(KEY_1, KEY_2)))
+        .addEqualityGroup(
+            MetricDescriptor.create(
+                METRIC_NAME_1,
+                DESCRIPTION,
+                UNIT,
+                Type.CUMULATIVE_DISTRIBUTION,
+                Arrays.asList(KEY_1)))
+        .testEquals();
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/MetricRegistryTest.java b/api/src/test/java/io/opencensus/metrics/MetricRegistryTest.java
new file mode 100644
index 00000000..49e8ce02
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/MetricRegistryTest.java
@@ -0,0 +1,185 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import io.opencensus.common.ToDoubleFunction;
+import io.opencensus.common.ToLongFunction;
+import java.util.LinkedHashMap;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link MetricRegistry}. */
+@RunWith(JUnit4.class)
+public class MetricRegistryTest {
+  @Rule public ExpectedException thrown = ExpectedException.none();
+
+  private final MetricRegistry metricRegistry =
+      MetricsComponent.newNoopMetricsComponent().getMetricRegistry();
+
+  @Test
+  public void addDoubleGauge_NullName() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addDoubleGauge(
+        null,
+        "description",
+        "1",
+        new LinkedHashMap<LabelKey, LabelValue>(),
+        null,
+        new ToDoubleFunction<Object>() {
+          @Override
+          public double applyAsDouble(Object value) {
+            return 5.0;
+          }
+        });
+  }
+
+  @Test
+  public void addDoubleGauge_NullDescription() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addDoubleGauge(
+        "name",
+        null,
+        "1",
+        new LinkedHashMap<LabelKey, LabelValue>(),
+        null,
+        new ToDoubleFunction<Object>() {
+          @Override
+          public double applyAsDouble(Object value) {
+            return 5.0;
+          }
+        });
+  }
+
+  @Test
+  public void addDoubleGauge_NullUnit() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addDoubleGauge(
+        "name",
+        "description",
+        null,
+        new LinkedHashMap<LabelKey, LabelValue>(),
+        null,
+        new ToDoubleFunction<Object>() {
+          @Override
+          public double applyAsDouble(Object value) {
+            return 5.0;
+          }
+        });
+  }
+
+  @Test
+  public void addDoubleGauge_NullLabels() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addDoubleGauge(
+        "name",
+        "description",
+        "1",
+        null,
+        null,
+        new ToDoubleFunction<Object>() {
+          @Override
+          public double applyAsDouble(Object value) {
+            return 5.0;
+          }
+        });
+  }
+
+  @Test
+  public void addDoubleGauge_NullFunction() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addDoubleGauge(
+        "name", "description", "1", new LinkedHashMap<LabelKey, LabelValue>(), null, null);
+  }
+
+  @Test
+  public void addLongGauge_NullName() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addLongGauge(
+        null,
+        "description",
+        "1",
+        new LinkedHashMap<LabelKey, LabelValue>(),
+        null,
+        new ToLongFunction<Object>() {
+          @Override
+          public long applyAsLong(Object value) {
+            return 5;
+          }
+        });
+  }
+
+  @Test
+  public void addLongGauge_NullDescription() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addLongGauge(
+        "name",
+        null,
+        "1",
+        new LinkedHashMap<LabelKey, LabelValue>(),
+        null,
+        new ToLongFunction<Object>() {
+          @Override
+          public long applyAsLong(Object value) {
+            return 5;
+          }
+        });
+  }
+
+  @Test
+  public void addLongGauge_NullUnit() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addLongGauge(
+        "name",
+        "description",
+        null,
+        new LinkedHashMap<LabelKey, LabelValue>(),
+        null,
+        new ToLongFunction<Object>() {
+          @Override
+          public long applyAsLong(Object value) {
+            return 5;
+          }
+        });
+  }
+
+  @Test
+  public void addLongGauge_NullLabels() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addLongGauge(
+        "name",
+        "description",
+        "1",
+        null,
+        null,
+        new ToLongFunction<Object>() {
+          @Override
+          public long applyAsLong(Object value) {
+            return 5;
+          }
+        });
+  }
+
+  @Test
+  public void addLongGauge_NullFunction() {
+    thrown.expect(NullPointerException.class);
+    metricRegistry.addLongGauge(
+        "name", "description", "1", new LinkedHashMap<LabelKey, LabelValue>(), null, null);
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/MetricTest.java b/api/src/test/java/io/opencensus/metrics/MetricTest.java
new file mode 100644
index 00000000..37deed4b
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/MetricTest.java
@@ -0,0 +1,116 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.testing.EqualsTester;
+import io.opencensus.common.Timestamp;
+import io.opencensus.metrics.MetricDescriptor.Type;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link Metric}. */
+@RunWith(JUnit4.class)
+public class MetricTest {
+
+  @Rule public final ExpectedException thrown = ExpectedException.none();
+
+  private static final String METRIC_NAME_1 = "metric1";
+  private static final String METRIC_NAME_2 = "metric2";
+  private static final String DESCRIPTION = "Metric description.";
+  private static final String UNIT = "kb/s";
+  private static final LabelKey KEY_1 = LabelKey.create("key1", "some key");
+  private static final LabelKey KEY_2 = LabelKey.create("key1", "some other key");
+  private static final MetricDescriptor METRIC_DESCRIPTOR_1 =
+      MetricDescriptor.create(
+          METRIC_NAME_1, DESCRIPTION, UNIT, Type.GAUGE_DOUBLE, Arrays.asList(KEY_1, KEY_2));
+  private static final MetricDescriptor METRIC_DESCRIPTOR_2 =
+      MetricDescriptor.create(
+          METRIC_NAME_2, DESCRIPTION, UNIT, Type.CUMULATIVE_INT64, Arrays.asList(KEY_1));
+  private static final LabelValue LABEL_VALUE_1 = LabelValue.create("value1");
+  private static final LabelValue LABEL_VALUE_2 = LabelValue.create("value1");
+  private static final LabelValue LABEL_VALUE_EMPTY = LabelValue.create("");
+  private static final Value VALUE_LONG = Value.longValue(12345678);
+  private static final Value VALUE_DOUBLE_1 = Value.doubleValue(-345.77);
+  private static final Value VALUE_DOUBLE_2 = Value.doubleValue(133.79);
+  private static final Timestamp TIMESTAMP_1 = Timestamp.fromMillis(1000);
+  private static final Timestamp TIMESTAMP_2 = Timestamp.fromMillis(2000);
+  private static final Timestamp TIMESTAMP_3 = Timestamp.fromMillis(3000);
+  private static final Point POINT_1 = Point.create(VALUE_DOUBLE_1, TIMESTAMP_2);
+  private static final Point POINT_2 = Point.create(VALUE_DOUBLE_2, TIMESTAMP_3);
+  private static final Point POINT_3 = Point.create(VALUE_LONG, TIMESTAMP_3);
+  private static final TimeSeries GAUGE_TIME_SERIES_1 =
+      TimeSeries.create(Arrays.asList(LABEL_VALUE_1, LABEL_VALUE_2), Arrays.asList(POINT_1), null);
+  private static final TimeSeries GAUGE_TIME_SERIES_2 =
+      TimeSeries.create(Arrays.asList(LABEL_VALUE_1, LABEL_VALUE_2), Arrays.asList(POINT_2), null);
+  private static final TimeSeries CUMULATIVE_TIME_SERIES =
+      TimeSeries.create(Arrays.asList(LABEL_VALUE_EMPTY), Arrays.asList(POINT_3), TIMESTAMP_1);
+
+  @Test
+  public void testGet() {
+    Metric metric =
+        Metric.create(METRIC_DESCRIPTOR_1, Arrays.asList(GAUGE_TIME_SERIES_1, GAUGE_TIME_SERIES_2));
+    assertThat(metric.getMetricDescriptor()).isEqualTo(METRIC_DESCRIPTOR_1);
+    assertThat(metric.getTimeSeriesList())
+        .containsExactly(GAUGE_TIME_SERIES_1, GAUGE_TIME_SERIES_2)
+        .inOrder();
+  }
+
+  @Test
+  public void typeMismatch_GaugeDouble_Long() {
+    typeMismatch(
+        METRIC_DESCRIPTOR_1,
+        Arrays.asList(CUMULATIVE_TIME_SERIES),
+        String.format("Type mismatch: %s, %s.", Type.GAUGE_DOUBLE, "ValueLong"));
+  }
+
+  @Test
+  public void typeMismatch_CumulativeInt64_Double() {
+    typeMismatch(
+        METRIC_DESCRIPTOR_2,
+        Arrays.asList(GAUGE_TIME_SERIES_1),
+        String.format("Type mismatch: %s, %s.", Type.CUMULATIVE_INT64, "ValueDouble"));
+  }
+
+  private void typeMismatch(
+      MetricDescriptor metricDescriptor, List<TimeSeries> timeSeriesList, String errorMessage) {
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage(errorMessage);
+    Metric.create(metricDescriptor, timeSeriesList);
+  }
+
+  @Test
+  public void testEquals() {
+    new EqualsTester()
+        .addEqualityGroup(
+            Metric.create(
+                METRIC_DESCRIPTOR_1, Arrays.asList(GAUGE_TIME_SERIES_1, GAUGE_TIME_SERIES_2)),
+            Metric.create(
+                METRIC_DESCRIPTOR_1, Arrays.asList(GAUGE_TIME_SERIES_1, GAUGE_TIME_SERIES_2)))
+        .addEqualityGroup(Metric.create(METRIC_DESCRIPTOR_1, Collections.<TimeSeries>emptyList()))
+        .addEqualityGroup(Metric.create(METRIC_DESCRIPTOR_2, Arrays.asList(CUMULATIVE_TIME_SERIES)))
+        .addEqualityGroup(Metric.create(METRIC_DESCRIPTOR_2, Collections.<TimeSeries>emptyList()))
+        .testEquals();
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/MetricsComponentTest.java b/api/src/test/java/io/opencensus/metrics/MetricsComponentTest.java
new file mode 100644
index 00000000..1c4e70f7
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/MetricsComponentTest.java
@@ -0,0 +1,40 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import io.opencensus.metrics.export.ExportComponent;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link MetricsComponent}. */
+@RunWith(JUnit4.class)
+public class MetricsComponentTest {
+  @Test
+  public void defaultExportComponent() {
+    assertThat(MetricsComponent.newNoopMetricsComponent().getExportComponent())
+        .isInstanceOf(ExportComponent.newNoopExportComponent().getClass());
+  }
+
+  @Test
+  public void defaultMetricRegistry() {
+    assertThat(MetricsComponent.newNoopMetricsComponent().getMetricRegistry())
+        .isInstanceOf(MetricRegistry.newNoopMetricRegistry().getClass());
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/MetricsTest.java b/api/src/test/java/io/opencensus/metrics/MetricsTest.java
new file mode 100644
index 00000000..9e0eee1f
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/MetricsTest.java
@@ -0,0 +1,71 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import io.opencensus.metrics.export.ExportComponent;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link Metrics}. */
+@RunWith(JUnit4.class)
+public class MetricsTest {
+  @Rule public ExpectedException thrown = ExpectedException.none();
+
+  @Test
+  public void loadMetricsComponent_UsesProvidedClassLoader() {
+    final RuntimeException toThrow = new RuntimeException("UseClassLoader");
+    thrown.expect(RuntimeException.class);
+    thrown.expectMessage("UseClassLoader");
+    Metrics.loadMetricsComponent(
+        new ClassLoader() {
+          @Override
+          public Class<?> loadClass(String name) {
+            throw toThrow;
+          }
+        });
+  }
+
+  @Test
+  public void loadMetricsComponent_IgnoresMissingClasses() {
+    ClassLoader classLoader =
+        new ClassLoader() {
+          @Override
+          public Class<?> loadClass(String name) throws ClassNotFoundException {
+            throw new ClassNotFoundException();
+          }
+        };
+    assertThat(Metrics.loadMetricsComponent(classLoader).getClass().getName())
+        .isEqualTo("io.opencensus.metrics.MetricsComponent$NoopMetricsComponent");
+  }
+
+  @Test
+  public void defaultExportComponent() {
+    assertThat(Metrics.getExportComponent())
+        .isInstanceOf(ExportComponent.newNoopExportComponent().getClass());
+  }
+
+  @Test
+  public void defaultMetricRegistry() {
+    assertThat(Metrics.getMetricRegistry())
+        .isInstanceOf(MetricRegistry.newNoopMetricRegistry().getClass());
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/PointTest.java b/api/src/test/java/io/opencensus/metrics/PointTest.java
new file mode 100644
index 00000000..cb6175c1
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/PointTest.java
@@ -0,0 +1,68 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.testing.EqualsTester;
+import io.opencensus.common.Timestamp;
+import io.opencensus.metrics.Distribution.Bucket;
+import java.util.Arrays;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link Point}. */
+@RunWith(JUnit4.class)
+public class PointTest {
+
+  private static final Value DOUBLE_VALUE = Value.doubleValue(55.5);
+  private static final Value LONG_VALUE = Value.longValue(9876543210L);
+  private static final Value DISTRIBUTION_VALUE =
+      Value.distributionValue(
+          Distribution.create(
+              6.6,
+              10,
+              678.54,
+              Arrays.asList(-1.0, 0.0, 1.0),
+              Arrays.asList(
+                  Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4))));
+  private static final Timestamp TIMESTAMP_1 = Timestamp.create(1, 2);
+  private static final Timestamp TIMESTAMP_2 = Timestamp.create(3, 4);
+  private static final Timestamp TIMESTAMP_3 = Timestamp.create(5, 6);
+
+  @Test
+  public void testGet() {
+    Point point = Point.create(DOUBLE_VALUE, TIMESTAMP_1);
+    assertThat(point.getValue()).isEqualTo(DOUBLE_VALUE);
+    assertThat(point.getTimestamp()).isEqualTo(TIMESTAMP_1);
+  }
+
+  @Test
+  public void testEquals() {
+    new EqualsTester()
+        .addEqualityGroup(
+            Point.create(DOUBLE_VALUE, TIMESTAMP_1), Point.create(DOUBLE_VALUE, TIMESTAMP_1))
+        .addEqualityGroup(Point.create(LONG_VALUE, TIMESTAMP_1))
+        .addEqualityGroup(Point.create(LONG_VALUE, TIMESTAMP_2))
+        .addEqualityGroup(
+            Point.create(DISTRIBUTION_VALUE, TIMESTAMP_2),
+            Point.create(DISTRIBUTION_VALUE, TIMESTAMP_2))
+        .addEqualityGroup(Point.create(DISTRIBUTION_VALUE, TIMESTAMP_3))
+        .testEquals();
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/TimeSeriesTest.java b/api/src/test/java/io/opencensus/metrics/TimeSeriesTest.java
new file mode 100644
index 00000000..07dff97d
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/TimeSeriesTest.java
@@ -0,0 +1,116 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.testing.EqualsTester;
+import io.opencensus.common.Timestamp;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import org.hamcrest.CoreMatchers;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link TimeSeries}. */
+@RunWith(JUnit4.class)
+public class TimeSeriesTest {
+
+  @Rule public ExpectedException thrown = ExpectedException.none();
+
+  private static final LabelValue LABEL_VALUE_1 = LabelValue.create("value1");
+  private static final LabelValue LABEL_VALUE_2 = LabelValue.create("value2");
+  private static final Value VALUE_LONG = Value.longValue(12345678);
+  private static final Value VALUE_DOUBLE = Value.doubleValue(-345.77);
+  private static final Timestamp TIMESTAMP_1 = Timestamp.fromMillis(1000);
+  private static final Timestamp TIMESTAMP_2 = Timestamp.fromMillis(2000);
+  private static final Timestamp TIMESTAMP_3 = Timestamp.fromMillis(3000);
+  private static final Point POINT_1 = Point.create(VALUE_DOUBLE, TIMESTAMP_2);
+  private static final Point POINT_2 = Point.create(VALUE_LONG, TIMESTAMP_3);
+
+  @Test
+  public void testGet_TimeSeries() {
+    TimeSeries cumulativeTimeSeries =
+        TimeSeries.create(
+            Arrays.asList(LABEL_VALUE_1, LABEL_VALUE_2), Arrays.asList(POINT_1), TIMESTAMP_1);
+    assertThat(cumulativeTimeSeries.getStartTimestamp()).isEqualTo(TIMESTAMP_1);
+    assertThat(cumulativeTimeSeries.getLabelValues())
+        .containsExactly(LABEL_VALUE_1, LABEL_VALUE_2)
+        .inOrder();
+    assertThat(cumulativeTimeSeries.getPoints()).containsExactly(POINT_1).inOrder();
+  }
+
+  @Test
+  public void create_WithNullLabelValueList() {
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage(CoreMatchers.equalTo("labelValues"));
+    TimeSeries.create(null, Collections.<Point>emptyList(), TIMESTAMP_1);
+  }
+
+  @Test
+  public void create_WithNullLabelValue() {
+    List<LabelValue> labelValues = Arrays.asList(LABEL_VALUE_1, null);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage(CoreMatchers.equalTo("labelValue"));
+    TimeSeries.create(labelValues, Collections.<Point>emptyList(), TIMESTAMP_1);
+  }
+
+  @Test
+  public void create_WithNullPointList() {
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage(CoreMatchers.equalTo("points"));
+    TimeSeries.create(Collections.<LabelValue>emptyList(), null, TIMESTAMP_1);
+  }
+
+  @Test
+  public void create_WithNullPoint() {
+    List<Point> points = Arrays.asList(POINT_1, null);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage(CoreMatchers.equalTo("point"));
+    TimeSeries.create(Collections.<LabelValue>emptyList(), points, TIMESTAMP_1);
+  }
+
+  @Test
+  public void testEquals() {
+    new EqualsTester()
+        .addEqualityGroup(
+            TimeSeries.create(
+                Arrays.asList(LABEL_VALUE_1, LABEL_VALUE_2), Arrays.asList(POINT_1), TIMESTAMP_1),
+            TimeSeries.create(
+                Arrays.asList(LABEL_VALUE_1, LABEL_VALUE_2), Arrays.asList(POINT_1), TIMESTAMP_1))
+        .addEqualityGroup(
+            TimeSeries.create(
+                Arrays.asList(LABEL_VALUE_1, LABEL_VALUE_2), Arrays.asList(POINT_1), null),
+            TimeSeries.create(
+                Arrays.asList(LABEL_VALUE_1, LABEL_VALUE_2), Arrays.asList(POINT_1), null))
+        .addEqualityGroup(
+            TimeSeries.create(
+                Arrays.asList(LABEL_VALUE_1, LABEL_VALUE_2), Arrays.asList(POINT_1), TIMESTAMP_2))
+        .addEqualityGroup(
+            TimeSeries.create(Arrays.asList(LABEL_VALUE_1), Arrays.asList(POINT_1), TIMESTAMP_2))
+        .addEqualityGroup(
+            TimeSeries.create(Arrays.asList(LABEL_VALUE_1), Arrays.asList(POINT_2), TIMESTAMP_2))
+        .addEqualityGroup(
+            TimeSeries.create(
+                Arrays.asList(LABEL_VALUE_1), Arrays.asList(POINT_1, POINT_2), TIMESTAMP_2))
+        .testEquals();
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/ValueTest.java b/api/src/test/java/io/opencensus/metrics/ValueTest.java
new file mode 100644
index 00000000..63430b28
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/ValueTest.java
@@ -0,0 +1,128 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.testing.EqualsTester;
+import io.opencensus.common.Function;
+import io.opencensus.common.Functions;
+import io.opencensus.metrics.Distribution.Bucket;
+import io.opencensus.metrics.Value.ValueDistribution;
+import io.opencensus.metrics.Value.ValueDouble;
+import io.opencensus.metrics.Value.ValueLong;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link Value}. */
+@RunWith(JUnit4.class)
+public class ValueTest {
+
+  private static final Distribution DISTRIBUTION =
+      Distribution.create(
+          10,
+          10,
+          1,
+          Arrays.asList(-5.0, 0.0, 5.0),
+          Arrays.asList(Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4)));
+
+  @Test
+  public void createAndGet_ValueDouble() {
+    Value value = Value.doubleValue(-34.56);
+    assertThat(value).isInstanceOf(ValueDouble.class);
+    assertThat(((ValueDouble) value).getValue()).isEqualTo(-34.56);
+  }
+
+  @Test
+  public void createAndGet_ValueLong() {
+    Value value = Value.longValue(123456789);
+    assertThat(value).isInstanceOf(ValueLong.class);
+    assertThat(((ValueLong) value).getValue()).isEqualTo(123456789);
+  }
+
+  @Test
+  public void createAndGet_ValueDistribution() {
+    Value value = Value.distributionValue(DISTRIBUTION);
+    assertThat(value).isInstanceOf(ValueDistribution.class);
+    assertThat(((ValueDistribution) value).getValue()).isEqualTo(DISTRIBUTION);
+  }
+
+  @Test
+  public void testEquals() {
+    new EqualsTester()
+        .addEqualityGroup(Value.doubleValue(1.0), Value.doubleValue(1.0))
+        .addEqualityGroup(Value.doubleValue(2.0))
+        .addEqualityGroup(Value.longValue(1L))
+        .addEqualityGroup(Value.longValue(2L))
+        .addEqualityGroup(
+            Value.distributionValue(
+                Distribution.create(
+                    -7,
+                    10,
+                    23.456,
+                    Arrays.asList(-5.0, 0.0, 5.0),
+                    Arrays.asList(
+                        Bucket.create(3), Bucket.create(1), Bucket.create(2), Bucket.create(4)))))
+        .testEquals();
+  }
+
+  @Test
+  public void testMatch() {
+    List<Value> values =
+        Arrays.asList(
+            ValueDouble.create(1.0), ValueLong.create(-1), ValueDistribution.create(DISTRIBUTION));
+    List<Number> expected =
+        Arrays.<Number>asList(1.0, -1L, 10.0, 10L, 1.0, -5.0, 0.0, 5.0, 3L, 1L, 2L, 4L);
+    final List<Number> actual = new ArrayList<Number>();
+    for (Value value : values) {
+      value.match(
+          new Function<Double, Object>() {
+            @Override
+            public Object apply(Double arg) {
+              actual.add(arg);
+              return null;
+            }
+          },
+          new Function<Long, Object>() {
+            @Override
+            public Object apply(Long arg) {
+              actual.add(arg);
+              return null;
+            }
+          },
+          new Function<Distribution, Object>() {
+            @Override
+            public Object apply(Distribution arg) {
+              actual.add(arg.getMean());
+              actual.add(arg.getCount());
+              actual.add(arg.getSumOfSquaredDeviations());
+              actual.addAll(arg.getBucketBoundaries());
+              for (Bucket bucket : arg.getBuckets()) {
+                actual.add(bucket.getCount());
+              }
+              return null;
+            }
+          },
+          Functions.throwAssertionError());
+    }
+    assertThat(actual).containsExactlyElementsIn(expected).inOrder();
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/export/ExportComponentTest.java b/api/src/test/java/io/opencensus/metrics/export/ExportComponentTest.java
new file mode 100644
index 00000000..15c6e883
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/export/ExportComponentTest.java
@@ -0,0 +1,33 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics.export;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link ExportComponent}. */
+@RunWith(JUnit4.class)
+public class ExportComponentTest {
+  @Test
+  public void defaultMetricExporter() {
+    assertThat(ExportComponent.newNoopExportComponent().getMetricProducerManager())
+        .isInstanceOf(MetricProducerManager.class);
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/export/MetricProducerManagerTest.java b/api/src/test/java/io/opencensus/metrics/export/MetricProducerManagerTest.java
new file mode 100644
index 00000000..07854927
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/export/MetricProducerManagerTest.java
@@ -0,0 +1,81 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics.export;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import io.opencensus.metrics.MetricProducer;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+/** Unit tests for {@link MetricProducerManager}. */
+@RunWith(JUnit4.class)
+public class MetricProducerManagerTest {
+  private final MetricProducerManager metricProducerManager =
+      MetricProducerManager.newNoopMetricProducerManager();
+  @Mock private MetricProducer metricProducer;
+
+  @Rule public final ExpectedException thrown = ExpectedException.none();
+
+  @Before
+  public void setUp() {
+    MockitoAnnotations.initMocks(this);
+  }
+
+  @Test
+  public void add_DisallowsNull() {
+    thrown.expect(NullPointerException.class);
+    metricProducerManager.add(null);
+  }
+
+  @Test
+  public void add() {
+    metricProducerManager.add(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
+  }
+
+  @Test
+  public void addAndRemove() {
+    metricProducerManager.add(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
+    metricProducerManager.remove(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
+  }
+
+  @Test
+  public void remove_DisallowsNull() {
+    thrown.expect(NullPointerException.class);
+    metricProducerManager.remove(null);
+  }
+
+  @Test
+  public void remove_FromEmpty() {
+    metricProducerManager.remove(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
+  }
+
+  @Test
+  public void getAllMetricProducer_empty() {
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
+  }
+}