[OTLP] Update CHANGELOG

Add entry for #6567.

Author: martincostello

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md

@@ -7,6 +7,10 @@ Notes](../../RELEASENOTES.md).
 
 ## Unreleased
 
+* Fixed an issue where OTLP export of some metrics payloads to a some OTLP
+  receivers were rejected with an HTTP 400 response due to gRPC protocol errors.
+  ([#6567](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6567))
+
 ## 1.13.0
 
 Released 2025-Oct-01