fix: convert Prometheus histograms to non-cumulative otel histogram

CHANGELOG.md

@@ -24,6 +24,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Fixed
 
 - Use `context.Background()` as default context instead of nil in `go.opentelemetry.io/contrib/bridges/otellogr`. (#6527)
+- Convert Prometheus histogram buckets to non-cumulative otel histogram buckets in `go.opentelemetry.io/contrib/bridges/prometheus`. (#6685)
 - Don't start spans that never end for filtered out gRPC stats handler in `go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc`. (#6695)
 
 <!-- Released section -->

bridges/prometheus/producer.go

@@ -296,22 +296,23 @@ func convertBuckets(buckets []*dto.Bucket, sampleCount uint64) ([]float64, []uin
 		bucketCounts = make([]uint64, len(buckets)+1)
 	}
 	exemplars := make([]metricdata.Exemplar[float64], 0)
+	var previousCount uint64
 	for i, bucket := range buckets {
 		// The last bound may be the +Inf bucket, which is implied in OTel, but
 		// is explicit in Prometheus. Skip the last boundary if it is the +Inf
 		// bound.
 		if bound := bucket.GetUpperBound(); !math.IsInf(bound, +1) {
 			bounds[i] = bound
 		}
-		bucketCounts[i] = bucket.GetCumulativeCount()
+		previousCount, bucketCounts[i] = bucket.GetCumulativeCount(), bucket.GetCumulativeCount()-previousCount
 		if ex := bucket.GetExemplar(); ex != nil {
 			exemplars = append(exemplars, convertExemplar(ex))
 		}
 	}
 	if !hasInf {
 		// The Inf bucket was missing, so set the last bucket counts to the
 		// overall count
-		bucketCounts[len(bucketCounts)-1] = sampleCount
+		bucketCounts[len(bucketCounts)-1] = sampleCount - previousCount
 	}
 	return bounds, bucketCounts, exemplars
 }

bridges/prometheus/producer_test.go

@@ -229,6 +229,44 @@ func TestProduce(t *testing.T) {
 				},
 			}},
 		},
+		{
+			name: "histogram cumulative values to non-cumulative",
+			testFn: func(reg *prometheus.Registry) {
+				metric := prometheus.NewHistogram(prometheus.HistogramOpts{
+					Name: "test_histogram_metric",
+					Help: "A histogram metric for testing",
+					ConstLabels: prometheus.Labels(map[string]string{
+						"foo": "bar",
+					}),
+				})
+				reg.MustRegister(metric)
+				metric.Observe(0.01)
+			},
+			expected: []metricdata.ScopeMetrics{{
+				Scope: instrumentation.Scope{
+					Name: scopeName,
+				},
+				Metrics: []metricdata.Metrics{
+					{
+						Name:        "test_histogram_metric",
+						Description: "A histogram metric for testing",
+						Data: metricdata.Histogram[float64]{
+							Temporality: metricdata.CumulativeTemporality,
+							DataPoints: []metricdata.HistogramDataPoint[float64]{
+								{
+									Count:        1,
+									Sum:          0.01,
+									Bounds:       []float64{0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10},
+									BucketCounts: []uint64{0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+									Attributes:   attribute.NewSet(attribute.String("foo", "bar")),
+									Exemplars:    []metricdata.Exemplar[float64]{},
+								},
+							},
+						},
+					},
+				},
+			}},
+		},
 		{
 			name: "histogram with exemplar",
 			testFn: func(reg *prometheus.Registry) {