support write byte[] to bytes_value instead of array_value in TagWriter

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md

@@ -23,6 +23,10 @@ Notes](../../RELEASENOTES.md).
   now set the `TE=trailers` HTTP request header to improve interoperability.
   ([#6449](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6449))
 
+* Improved performance exporting `byte[]` attributes as native binary format
+  instead of arrays.
+  ([#6534](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6534))
+
 ## 1.12.0
 
 Released 2025-Apr-29

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

@@ -88,6 +88,20 @@ protected override bool TryWriteEmptyTag(ref OtlpTagWriterState state, string ke
         return true;
     }
 
+    protected override bool TryWriteByteArrayTag(ref OtlpTagWriterState state, string key, ReadOnlySpan<byte> value)
+    {
+        // Write KeyValue tag
+        state.WritePosition = ProtobufSerializer.WriteStringWithTag(state.Buffer, state.WritePosition, ProtobufOtlpCommonFieldNumberConstants.KeyValue_Key, key);
+
+        var serializedLengthSize = ProtobufSerializer.ComputeVarInt64Size((ulong)value.Length);
+
+        // length = value.Length + tagSize + length field size.
+        state.WritePosition = ProtobufSerializer.WriteTagAndLength(state.Buffer, state.WritePosition, value.Length + 1 + serializedLengthSize, ProtobufOtlpCommonFieldNumberConstants.KeyValue_Value, ProtobufWireType.LEN);
+        state.WritePosition = ProtobufSerializer.WriteByteArrayWithTag(state.Buffer, state.WritePosition, ProtobufOtlpCommonFieldNumberConstants.AnyValue_Bytes_Value, value);
+
+        return true;
+    }
+
     internal struct OtlpTagWriterState
     {
         public byte[] Buffer;

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

@@ -231,6 +231,17 @@ internal static int ComputeVarInt64Size(ulong value)
         return 10;
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    internal static int WriteByteArrayWithTag(byte[] buffer, int writePosition, int fieldNumber, ReadOnlySpan<byte> value)
+    {
+        writePosition = WriteTag(buffer, writePosition, fieldNumber, ProtobufWireType.LEN);
+        writePosition = WriteLength(buffer, writePosition, value.Length);
+        value.CopyTo(buffer.AsSpan(writePosition));
+
+        writePosition += value.Length;
+        return writePosition;
+    }
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     internal static int WriteStringWithTag(byte[] buffer, int writePosition, int fieldNumber, string value)
     {

src/Shared/TagWriter/JsonStringArrayTagWriter.cs

@@ -25,6 +25,8 @@ protected sealed override void WriteArrayTag(ref TTagState writer, string key, r
 
     protected abstract void WriteArrayTag(ref TTagState writer, string key, ArraySegment<byte> arrayUtf8JsonBytes);
 
+    protected override bool TryWriteByteArrayTag(ref TTagState consoleTag, string key, ReadOnlySpan<byte> value) => false;
+
     internal readonly struct JsonArrayTagWriterState(MemoryStream stream, Utf8JsonWriter writer)
     {
         public MemoryStream Stream { get; } = stream;

src/Shared/TagWriter/TagWriter.cs

@@ -67,6 +67,11 @@ public bool TryWriteTag(
                 this.WriteFloatingPointTag(ref state, key, Convert.ToDouble(value, CultureInfo.InvariantCulture));
                 break;
             case Array array:
+                if (value.GetType() == typeof(byte[]) && this.TryWriteByteArrayTag(ref state, key, ((byte[])value).AsSpan()))
+                {
+                    return true;
+                }
+
                 try
                 {
                     this.WriteArrayTagInternal(ref state, key, array, tagValueMaxLength);
@@ -119,6 +124,8 @@ public bool TryWriteTag(
 
     protected abstract bool TryWriteEmptyTag(ref TTagState state, string key, object? value);
 
+    protected abstract bool TryWriteByteArrayTag(ref TTagState state, string key, ReadOnlySpan<byte> value);
+
     protected abstract void WriteIntegralTag(ref TTagState state, string key, long value);
 
     protected abstract void WriteFloatingPointTag(ref TTagState state, string key, double value);

test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpAttributeTests.cs

@@ -61,14 +61,23 @@ public void IntegralTypesSupported(object value)
         switch (value)
         {
             case Array array:
-                Assert.Equal(OtlpCommon.AnyValue.ValueOneofCase.ArrayValue, attribute.Value.ValueCase);
-                var expectedArray = new long[array.Length];
-                for (var i = 0; i < array.Length; i++)
+                if (value.GetType() == typeof(byte[]))
+                {
+                    Assert.Equal(OtlpCommon.AnyValue.ValueOneofCase.BytesValue, attribute.Value.ValueCase);
+                    Assert.Equal((byte[])value, attribute.Value.BytesValue.ToByteArray());
+                }
+                else
                 {
-                    expectedArray[i] = Convert.ToInt64(array.GetValue(i), CultureInfo.InvariantCulture);
+                    Assert.Equal(OtlpCommon.AnyValue.ValueOneofCase.ArrayValue, attribute.Value.ValueCase);
+                    var expectedArray = new long[array.Length];
+                    for (var i = 0; i < array.Length; i++)
+                    {
+                        expectedArray[i] = Convert.ToInt64(array.GetValue(i), CultureInfo.InvariantCulture);
+                    }
+
+                    Assert.Equal(expectedArray, attribute.Value.ArrayValue.Values.Select(x => x.IntValue));
                 }
 
-                Assert.Equal(expectedArray, attribute.Value.ArrayValue.Values.Select(x => x.IntValue));
                 break;
             default:
                 Assert.Equal(OtlpCommon.AnyValue.ValueOneofCase.IntValue, attribute.Value.ValueCase);

test/OpenTelemetry.Tests/Internal/JsonStringArrayTagWriterTests.cs

@@ -198,6 +198,8 @@ protected override bool TryWriteEmptyTag(ref Tag state, string key, object? valu
             throw new NotImplementedException();
         }
 
+        protected override bool TryWriteByteArrayTag(ref Tag consoleTag, string key, ReadOnlySpan<byte> value) => false;
+
         public struct Tag
         {
             public string? Key;