otelcol.exporter.prometheus: Make histogram counts cumulative for Pro…

…metheus (#4196)

CHANGELOG.md

@@ -57,6 +57,8 @@ Main (unreleased)
 
 - Fixes several issues with statsd exporter. (@jcreixell, @marctc)
 
+- Fixes a bug in conversion of OpenTelemetry histograms when exported to Prometheus. (@grcevski)
+
 ### Other changes
 
 - Mongodb integration has been disabled for the time being due to licensing issues. (@jcreixell)

component/otelcol/exporter/prometheus/internal/convert/convert.go

@@ -10,6 +10,8 @@ package convert
 import (
 	"context"
 	"fmt"
+	"math"
+	"sort"
 	"strconv"
 	"sync"
 	"time"
@@ -458,11 +460,36 @@ func (conv *Converter) consumeHistogram(app storage.Appender, memResource *memor
 			}
 		}
 
+		// Sort the histogram by bounds ascending
+		bSize := int(math.Min(float64(dp.ExplicitBounds().Len()), float64(dp.BucketCounts().Len())))
+		buckets := make(map[float64]uint64, bSize)
+		bounds := make([]float64, 0, bSize)
+		for i := 0; i < dp.ExplicitBounds().Len() && i < dp.BucketCounts().Len(); i++ {
+			bound := dp.ExplicitBounds().At(i)
+			buckets[bound] = dp.BucketCounts().At(i)
+			bounds = append(bounds, bound)
+		}
+
+		sort.Float64s(bounds)
+
+		// Calculate cumulative count values. Prometheus expects cummulative bucket counts for histograms.
+		// This has nothing to do with temporality, it doesn't affect cummulative vs delta histograms, it
+		// simply matches the format of bucket counts expected by Prometheus.
+		var c uint64 = 0
+		for i := 0; i < len(bounds); i++ {
+			bound := bounds[i]
+			c += buckets[bound]
+			buckets[bound] = c
+		}
+
 		// Process the boundaries. The number of buckets = number of explicit
 		// bounds + 1.
 		for i := 0; i < dp.ExplicitBounds().Len() && i < dp.BucketCounts().Len(); i++ {
 			bound := dp.ExplicitBounds().At(i)
-			count := dp.BucketCounts().At(i)
+			count, ok := buckets[bound]
+			if !ok {
+				count = dp.BucketCounts().At(i)
+			}
 
 			bucketLabel := labels.Label{
 				Name:  model.BucketLabel,

component/otelcol/exporter/prometheus/internal/convert/convert_test.go

@@ -121,8 +121,41 @@ func TestConverter(t *testing.T) {
 				# TYPE test_metric_seconds histogram
 				test_metric_seconds_bucket{le="0.25"} 0
 				test_metric_seconds_bucket{le="0.5"} 111
-				test_metric_seconds_bucket{le="0.75"} 0
-				test_metric_seconds_bucket{le="1.0"} 222
+				test_metric_seconds_bucket{le="0.75"} 111
+				test_metric_seconds_bucket{le="1.0"} 333
+				test_metric_seconds_bucket{le="+Inf"} 333
+				test_metric_seconds_sum 100.0
+				test_metric_seconds_count 333
+			`,
+		},
+		{
+			name: "Histogram out-of-order bounds",
+			input: `{
+				"resource_metrics": [{
+					"scope_metrics": [{
+						"metrics": [{
+							"name": "test_metric_seconds",
+							"histogram": {
+								"aggregation_temporality": 2,
+								"data_points": [{
+									"start_time_unix_nano": 1000000000,
+									"time_unix_nano": 1000000000,
+									"count": 333,
+									"sum": 100,
+									"bucket_counts": [0, 111, 0, 222],
+									"explicit_bounds": [0.5, 1.0, 0.25, 0.75]
+								}]
+							}
+						}]
+					}]
+				}]
+			}`,
+			expect: `
+				# TYPE test_metric_seconds histogram
+				test_metric_seconds_bucket{le="0.25"} 0
+				test_metric_seconds_bucket{le="0.5"} 0
+				test_metric_seconds_bucket{le="0.75"} 222
+				test_metric_seconds_bucket{le="1.0"} 333
 				test_metric_seconds_bucket{le="+Inf"} 333
 				test_metric_seconds_sum 100.0
 				test_metric_seconds_count 333