[OTLP] Update CHANGELOG

Add entries for open-telemetry#6562.

Author: martincostello

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md

@@ -7,6 +7,12 @@ Notes](../../RELEASENOTES.md).
 
 ## Unreleased
 
+* Avoid allocating strings when logging gRPC export events unless required.
+  ([#6562](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6562))
+
+* Log export success when exporting telemetry using `http/protobuf`.
+  ([#6562](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6562))
+
 ## 1.13.0
 
 Released 2025-Oct-01