[OTLP] Use packed format for metric histograms

Use packed format for metric histograms.

Initial code written and tweaked by Copilot. Passes integration tests with 0.133.0 and 0.136.0 of the OpenTelemetry Collector.

Author: martincostello

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Serializer/ProtobufOtlpMetricSerializer.cs

@@ -292,14 +292,49 @@ private static int WriteMetric(byte[] buffer, int writePosition, Metric metric)
                             writePosition = ProtobufSerializer.WriteDoubleWithTag(buffer, writePosition, ProtobufOtlpMetricFieldNumberConstants.HistogramDataPoint_Max, max);
                         }
 
-                        foreach (var histogramMeasurement in metricPoint.GetHistogramBuckets())
+                        // Collect all counts (assuming GetHistogramBuckets() returns in order)
+                        var bucketCounts = new List<ulong>();
+
+                        foreach (var x in metricPoint.GetHistogramBuckets())
+                        {
+                            bucketCounts.Add((ulong)x.BucketCount);
+                        }
+
+                        // Write packed repeated fixed64 bucket_counts
+                        var fieldNumber = ProtobufOtlpMetricFieldNumberConstants.HistogramDataPoint_Bucket_Counts;
+                        var tag2 = (fieldNumber << 3) | 2; // wire type 2 for length-delimited
+                        writePosition = ProtobufSerializer.WriteVarInt32(buffer, writePosition, (uint)tag2);
+
+                        int length = bucketCounts.Count * 8;
+                        writePosition = ProtobufSerializer.WriteVarInt32(buffer, writePosition, (uint)length);
+
+                        foreach (var bucketCount in bucketCounts)
+                        {
+                            writePosition = ProtobufSerializer.WriteFixed64LittleEndianFormat(buffer, writePosition, bucketCount);
+                        }
+
+                        var explicitBounds = new List<double>();
+
+                        foreach (var x in metricPoint.GetHistogramBuckets())
+                        {
+                            if (x.ExplicitBound != double.PositiveInfinity)
+                            {
+                                explicitBounds.Add(x.ExplicitBound);
+                            }
+                        }
+
+                        if (explicitBounds.Count > 0)
                         {
-                            var bucketCount = (ulong)histogramMeasurement.BucketCount;
-                            writePosition = ProtobufSerializer.WriteFixed64WithTag(buffer, writePosition, ProtobufOtlpMetricFieldNumberConstants.HistogramDataPoint_Bucket_Counts, bucketCount);
+                            var boundsFieldNumber = ProtobufOtlpMetricFieldNumberConstants.HistogramDataPoint_Explicit_Bounds;
+                            var boundsTag = (boundsFieldNumber << 3) | 2; // wire type 2
+                            writePosition = ProtobufSerializer.WriteVarInt32(buffer, writePosition, (uint)boundsTag);
+
+                            int boundsLength = explicitBounds.Count * 8;
+                            writePosition = ProtobufSerializer.WriteVarInt32(buffer, writePosition, (uint)boundsLength);
 
-                            if (histogramMeasurement.ExplicitBound != double.PositiveInfinity)
+                            foreach (var bound in explicitBounds)
                             {
-                                writePosition = ProtobufSerializer.WriteDoubleWithTag(buffer, writePosition, ProtobufOtlpMetricFieldNumberConstants.HistogramDataPoint_Explicit_Bounds, histogramMeasurement.ExplicitBound);
+                                writePosition = ProtobufSerializer.WriteDouble(buffer, writePosition, bound);
                             }
                         }
 

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Serializer/ProtobufSerializer.cs

@@ -160,6 +160,10 @@ internal static int WriteDoubleWithTag(byte[] buffer, int writePosition, int fie
         return writePosition;
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    internal static int WriteDouble(byte[] buffer, int writePosition, double value)
+        => WriteFixed64LittleEndianFormat(buffer, writePosition, (ulong)BitConverter.DoubleToInt64Bits(value));
+
     /// <summary>
     /// Computes the number of bytes required to encode a 64-bit unsigned integer in Protocol Buffers' varint format.
     /// </summary>

test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/IntegrationTest/docker-compose.yml

@@ -19,7 +19,7 @@ services:
       "
 
   otel-collector:
-    image: otel/opentelemetry-collector:0.136.0@sha256:98fd3b410ae8a939be9588f1580c4b7c3da6ebba49f5363df4259a827aabb779
+    image: otel/opentelemetry-collector:0.133.0
     volumes:
       - ./test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/IntegrationTest:/cfg
     command: --config=/cfg/otel-collector-config.yaml