[otlp] Change serialization of null protobuf values

src/OpenTelemetry.Exporter.Console/Implementation/ConsoleTagWriter.cs

@@ -73,6 +73,13 @@ protected override void OnUnsupportedTagDropped(
         this.onUnsupportedTagDropped(tagKey, tagValueTypeFullName);
     }
 
+    protected override bool TryWriteEmptyTag(ref ConsoleTag consoleTag, string key, object? value)
+    {
+        consoleTag.Key = key;
+        consoleTag.Value = null;
+        return true;
+    }
+
     internal struct ConsoleTag
     {
         public string? Key;

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md

@@ -16,6 +16,11 @@ Notes](../../RELEASENOTES.md).
   `Activity.StatusDescription` exceeds 127 bytes.
   ([#6119](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6119))
 
+* Fixed incorrect log serialization of attributes with null values, causing
+  some backends to reject logs.
+  some backends to reject logs when using OTLP exporter to output protobuf.
+  ([#6149](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6149))
+
 ## 1.11.1
 
 Released 2025-Jan-22

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

@@ -81,6 +81,13 @@ protected override void OnUnsupportedTagDropped(
             tagValueTypeFullName,
             tagKey);
 
+    protected override bool TryWriteEmptyTag(ref OtlpTagWriterState state, string key, object? value)
+    {
+        state.WritePosition = ProtobufSerializer.WriteStringWithTag(state.Buffer, state.WritePosition, ProtobufOtlpCommonFieldNumberConstants.KeyValue_Key, key);
+        state.WritePosition = ProtobufSerializer.WriteTagAndLength(state.Buffer, state.WritePosition, 0, ProtobufOtlpCommonFieldNumberConstants.KeyValue_Value, ProtobufWireType.LEN);
+        return true;
+    }
+
     internal struct OtlpTagWriterState
     {
         public byte[] Buffer;

src/OpenTelemetry.Exporter.Zipkin/Implementation/ZipkinTagWriter.cs

@@ -64,4 +64,6 @@ protected override void OnUnsupportedTagDropped(
             tagValueTypeFullName,
             tagKey);
     }
+
+    protected override bool TryWriteEmptyTag(ref Utf8JsonWriter state, string key, object? value) => false;
 }

src/Shared/TagWriter/TagWriter.cs

@@ -36,7 +36,7 @@ public bool TryWriteTag(
     {
         if (value == null)
         {
-            return false;
+            return this.TryWriteEmptyTag(ref state, key, value);
         }
 
         switch (value)
@@ -117,6 +117,8 @@ public bool TryWriteTag(
         return true;
     }
 
+    protected abstract bool TryWriteEmptyTag(ref TTagState state, string key, object? 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

@@ -14,7 +14,13 @@ public class OtlpAttributeTests
     public void NullValueAttribute()
     {
         var kvp = new KeyValuePair<string, object?>("key", null);
-        Assert.False(TryTransformTag(kvp, out _));
+        Assert.True(TryTransformTag(kvp, out var attribute));
+        Assert.Equal(OtlpCommon.AnyValue.ValueOneofCase.None, attribute.Value.ValueCase);
+        Assert.False(attribute.Value.HasBoolValue);
+        Assert.False(attribute.Value.HasBytesValue);
+        Assert.False(attribute.Value.HasDoubleValue);
+        Assert.False(attribute.Value.HasIntValue);
+        Assert.False(attribute.Value.HasStringValue);
     }
 
     [Fact]

test/OpenTelemetry.Tests/Internal/JsonStringArrayTagWriterTests.cs

@@ -193,6 +193,11 @@ protected override void OnUnsupportedTagDropped(string tagKey, string tagValueTy
         {
         }
 
+        protected override bool TryWriteEmptyTag(ref Tag state, string key, object? value)
+        {
+            throw new NotImplementedException();
+        }
+
         public struct Tag
         {
             public string? Key;