Exporter/Stats/Stackdriver: Add 0 count for underflow/first bucket (census-instrumentation#1525)

* Add 0 count for underflow/first bucket

* Fix comments

Author: mayurkale22

exporters/stats/stackdriver/src/main/java/io/opencensus/exporter/stats/stackdriver/StackdriverExportUtils.java

@@ -124,7 +124,13 @@ public TypedValue apply(Summary arg) {
         @Override
         public BucketOptions apply(ExplicitOptions arg) {
           BucketOptions.Builder builder = BucketOptions.newBuilder();
-          builder.setExplicitBuckets(Explicit.newBuilder().addAllBounds(arg.getBucketBoundaries()));
+          Explicit.Builder explicitBuilder = Explicit.newBuilder();
+          // The first bucket bound should be 0.0 because the Metrics first bucket is
+          // [0, first_bound) but Stackdriver monitoring bucket bounds begin with -infinity
+          // (first bucket is (-infinity, 0))
+          explicitBuilder.addBounds(0.0);
+          explicitBuilder.addAllBounds(arg.getBucketBoundaries());
+          builder.setExplicitBuckets(explicitBuilder.build());
           return builder.build();
         }
       };
@@ -333,6 +339,9 @@ static BucketOptions createBucketOptions(
   // Convert a OpenCensus Buckets to a list of counts
   private static List<Long> createBucketCounts(List<Bucket> buckets) {
     List<Long> bucketCounts = new ArrayList<>();
+    // The first bucket (underflow bucket) should always be 0 count because the Metrics first bucket
+    // is [0, first_bound) but StackDriver distribution consists of an underflow bucket (number 0).
+    bucketCounts.add(0L);
     for (Bucket bucket : buckets) {
       bucketCounts.add(bucket.getCount());
     }

exporters/stats/stackdriver/src/test/java/io/opencensus/exporter/stats/stackdriver/StackdriverExportUtilsTest.java

@@ -119,6 +119,18 @@ public class StackdriverExportUtilsTest {
   private static final Value DISTRIBUTION_VALUE = Value.distributionValue(DISTRIBUTION);
   private static final Value SUMMARY_VALUE = Value.summaryValue(SUMMARY);
 
+  private static final io.opencensus.metrics.export.MetricDescriptor HISTOGRAM_METRIC_DESCRIPTOR =
+      io.opencensus.metrics.export.MetricDescriptor.create(
+          METRIC_NAME, METRIC_DESCRIPTION, METRIC_UNIT, Type.CUMULATIVE_DISTRIBUTION, LABEL_KEY);
+
+  private static final Point DISTRIBUTION_POINT = Point.create(DISTRIBUTION_VALUE, TIMESTAMP);
+  private static final io.opencensus.metrics.export.TimeSeries DISTRIBUTION_TIME_SERIES =
+      io.opencensus.metrics.export.TimeSeries.createWithOnePoint(
+          LABEL_VALUE, DISTRIBUTION_POINT, null);
+  private static final io.opencensus.metrics.export.Metric DISTRIBUTION_METRIC =
+      io.opencensus.metrics.export.Metric.createWithOneTimeSeries(
+          HISTOGRAM_METRIC_DESCRIPTOR, DISTRIBUTION_TIME_SERIES);
+
   @Test
   public void createLabelDescriptor() {
     assertThat(StackdriverExportUtils.createLabelDescriptor(LabelKey.create("key", "desc")))
@@ -216,7 +228,7 @@ public void createBucketOptions() {
         .isEqualTo(
             BucketOptions.newBuilder()
                 .setExplicitBuckets(
-                    Explicit.newBuilder().addAllBounds(Arrays.asList(1.0, 3.0, 5.0)))
+                    Explicit.newBuilder().addAllBounds(Arrays.asList(0.0, 1.0, 3.0, 5.0)))
                 .build());
   }
 
@@ -234,7 +246,7 @@ public void createDistribution() {
                 .setCount(3)
                 .setMean(0.6666666666666666)
                 .setBucketOptions(StackdriverExportUtils.createBucketOptions(BUCKET_OPTIONS))
-                .addAllBucketCounts(Arrays.asList(3L, 1L, 2L, 4L))
+                .addAllBucketCounts(Arrays.asList(0L, 3L, 1L, 2L, 4L))
                 .setSumOfSquaredDeviation(14)
                 .build());
   }
@@ -363,6 +375,32 @@ public void createTimeSeriesList_Cumulative() {
     assertThat(timeSeriesList).containsExactly(expectedTimeSeries);
   }
 
+  @Test
+  public void createTimeSeriesList_Distribution() {
+    List<TimeSeries> timeSeriesList =
+        StackdriverExportUtils.createTimeSeriesList(
+            DISTRIBUTION_METRIC, DEFAULT_RESOURCE, CUSTOM_OPENCENSUS_DOMAIN);
+
+    assertThat(timeSeriesList.size()).isEqualTo(1);
+    TimeSeries timeSeries = timeSeriesList.get(0);
+    assertThat(timeSeries.getPointsCount()).isEqualTo(1);
+    assertThat(timeSeries.getPoints(0).getValue().getDistributionValue())
+        .isEqualTo(
+            com.google.api.Distribution.newBuilder()
+                .setCount(3)
+                .setMean(0.6666666666666666)
+                .setBucketOptions(
+                    BucketOptions.newBuilder()
+                        .setExplicitBuckets(
+                            Explicit.newBuilder()
+                                .addAllBounds(Arrays.asList(0.0, 1.0, 3.0, 5.0))
+                                .build())
+                        .build())
+                .addAllBucketCounts(Arrays.asList(0L, 3L, 1L, 2L, 4L))
+                .setSumOfSquaredDeviation(14)
+                .build());
+  }
+
   @Test
   public void createTimeSeriesList_Gauge() {
     io.opencensus.metrics.export.Metric metric =