Enable exporting Histogram aggregation to OTLP metric (#1209)

* Add Count() to Histogram interface

* Fix copy/pasted comment for Count() interface

* Implement Histogram conversion to OTLP protobuf

* Add CHANGELOG message

* Filled in PR # for Changelog

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Added
+
+- OTLP Metric exporter supports Histogram aggregation. (#1209)
+
 ## [0.12.0] - 2020-09-24
 
 ### Added

exporters/otlp/internal/transform/metric.go

@@ -257,6 +257,13 @@ func Record(r export.Record) (*metricpb.Metric, error) {
 		}
 		return minMaxSumCount(r, mmsc)
 
+	case aggregation.HistogramKind:
+		h, ok := agg.(aggregation.Histogram)
+		if !ok {
+			return nil, fmt.Errorf("%w: %T", ErrIncompatibleAgg, agg)
+		}
+		return histogram(r, h)
+
 	case aggregation.SumKind:
 		s, ok := agg.(aggregation.Sum)
 		if !ok {
@@ -327,7 +334,7 @@ func scalar(record export.Record, num metric.Number, start, end time.Time) (*met
 }
 
 // minMaxSumCountValue returns the values of the MinMaxSumCount Aggregator
-// as discret values.
+// as discrete values.
 func minMaxSumCountValues(a aggregation.MinMaxSumCount) (min, max, sum metric.Number, count int64, err error) {
 	if min, err = a.Min(); err != nil {
 		return
@@ -383,6 +390,67 @@ func minMaxSumCount(record export.Record, a aggregation.MinMaxSumCount) (*metric
 	}, nil
 }
 
+func histogramValues(a aggregation.Histogram) (boundaries []float64, counts []float64, err error) {
+	var buckets aggregation.Buckets
+	if buckets, err = a.Histogram(); err != nil {
+		return
+	}
+	boundaries, counts = buckets.Boundaries, buckets.Counts
+	if len(counts) != len(boundaries)+1 {
+		err = ErrTransforming
+		return
+	}
+	return
+}
+
+// histogram transforms a Histogram Aggregator into an OTLP Metric.
+func histogram(record export.Record, a aggregation.Histogram) (*metricpb.Metric, error) {
+	desc := record.Descriptor()
+	labels := record.Labels()
+	boundaries, counts, err := histogramValues(a)
+	if err != nil {
+		return nil, err
+	}
+
+	count, err := a.Count()
+	if err != nil {
+		return nil, err
+	}
+
+	sum, err := a.Sum()
+	if err != nil {
+		return nil, err
+	}
+
+	buckets := make([]*metricpb.HistogramDataPoint_Bucket, len(counts))
+	for i := 0; i < len(counts); i++ {
+		buckets[i] = &metricpb.HistogramDataPoint_Bucket{
+			Count: uint64(counts[i]),
+		}
+	}
+
+	numKind := desc.NumberKind()
+	return &metricpb.Metric{
+		MetricDescriptor: &metricpb.MetricDescriptor{
+			Name:        desc.Name(),
+			Description: desc.Description(),
+			Unit:        string(desc.Unit()),
+			Type:        metricpb.MetricDescriptor_HISTOGRAM,
+		},
+		HistogramDataPoints: []*metricpb.HistogramDataPoint{
+			{
+				Labels:            stringKeyValues(labels.Iter()),
+				StartTimeUnixNano: toNanos(record.StartTime()),
+				TimeUnixNano:      toNanos(record.EndTime()),
+				Count:             uint64(count),
+				Sum:               sum.CoerceToFloat64(numKind),
+				Buckets:           buckets,
+				ExplicitBounds:    boundaries,
+			},
+		},
+	}, nil
+}
+
 // stringKeyValues transforms a label iterator into an OTLP StringKeyValues.
 func stringKeyValues(iter label.Iterator) []*commonpb.StringKeyValue {
 	l := iter.Len()

exporters/otlp/otlp_metric_test.go

@@ -33,7 +33,7 @@ import (
 	metricsdk "go.opentelemetry.io/otel/sdk/export/metric"
 	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/export/metric/metrictest"
-	"go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator/histogram"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/sum"
 	"go.opentelemetry.io/otel/sdk/resource"
 
@@ -107,6 +107,8 @@ var (
 	testInstA = resource.New(label.String("instance", "tester-a"))
 	testInstB = resource.New(label.String("instance", "tester-b"))
 
+	testHistogramBoundaries = []float64{2.0, 4.0, 8.0}
+
 	md = &metricpb.MetricDescriptor{
 		Name: "int64-count",
 		Type: metricpb.MetricDescriptor_INT64,
@@ -229,9 +231,9 @@ func TestValuerecorderMetricGroupingExport(t *testing.T) {
 						{
 							MetricDescriptor: &metricpb.MetricDescriptor{
 								Name: "valuerecorder",
-								Type: metricpb.MetricDescriptor_SUMMARY,
+								Type: metricpb.MetricDescriptor_HISTOGRAM,
 							},
-							SummaryDataPoints: []*metricpb.SummaryDataPoint{
+							HistogramDataPoints: []*metricpb.HistogramDataPoint{
 								{
 									Labels: []*commonpb.StringKeyValue{
 										{
@@ -243,20 +245,14 @@ func TestValuerecorderMetricGroupingExport(t *testing.T) {
 											Value: "test.com",
 										},
 									},
-									Count: 2,
-									Sum:   11,
-									PercentileValues: []*metricpb.SummaryDataPoint_ValueAtPercentile{
-										{
-											Percentile: 0.0,
-											Value:      1.0,
-										},
-										{
-											Percentile: 100.0,
-											Value:      10.0,
-										},
-									},
 									StartTimeUnixNano: startTime(),
 									TimeUnixNano:      pointTime(),
+									Count:             2,
+									Sum:               11,
+									ExplicitBounds:    testHistogramBoundaries,
+									Buckets: []*metricpb.HistogramDataPoint_Bucket{
+										{Count: 1}, {Count: 0}, {Count: 0}, {Count: 1},
+									},
 								},
 								{
 									Labels: []*commonpb.StringKeyValue{
@@ -269,17 +265,11 @@ func TestValuerecorderMetricGroupingExport(t *testing.T) {
 											Value: "test.com",
 										},
 									},
-									Count: 2,
-									Sum:   11,
-									PercentileValues: []*metricpb.SummaryDataPoint_ValueAtPercentile{
-										{
-											Percentile: 0.0,
-											Value:      1.0,
-										},
-										{
-											Percentile: 100.0,
-											Value:      10.0,
-										},
+									Count:          2,
+									Sum:            11,
+									ExplicitBounds: testHistogramBoundaries,
+									Buckets: []*metricpb.HistogramDataPoint_Bucket{
+										{Count: 1}, {Count: 0}, {Count: 0}, {Count: 1},
 									},
 									StartTimeUnixNano: startTime(),
 									TimeUnixNano:      pointTime(),
@@ -690,7 +680,7 @@ func runMetricExportTest(t *testing.T, exp *Exporter, rs []record, expected []me
 		case metric.CounterKind:
 			agg, ckpt = metrictest.Unslice2(sum.New(2))
 		default:
-			agg, ckpt = metrictest.Unslice2(minmaxsumcount.New(2, &desc))
+			agg, ckpt = metrictest.Unslice2(histogram.New(2, &desc, testHistogramBoundaries))
 		}
 
 		ctx := context.Background()

sdk/export/metric/aggregation/aggregation.go

@@ -40,7 +40,7 @@ type (
 		Sum() (metric.Number, error)
 	}
 
-	// Sum returns the number of values that were aggregated.
+	// Count returns the number of values that were aggregated.
 	Count interface {
 		Aggregation
 		Count() (int64, error)
@@ -95,6 +95,7 @@ type (
 	// Histogram returns the count of events in pre-determined buckets.
 	Histogram interface {
 		Aggregation
+		Count() (int64, error)
 		Sum() (metric.Number, error)
 		Histogram() (Buckets, error)
 	}