chore: note that user-agent cannot be overwritten by users anymore

experimental/CHANGELOG.md

@@ -11,6 +11,8 @@ All notable changes to experimental packages in this project will be documented
  * (user-facing) `headers` was intended for internal use has been removed from all exporters
  * (user-facing) `compression` was intended for internal use and has been removed from all exporters
  * (user-facing) `hostname` was intended for use in tests and is not used by any exporters, it will be removed in a future release
+* fix(exporter-*-otlp-*)!: ensure `User-Agent` header cannot be overwritten by the user [#4743](https://github.com/open-telemetry/opentelemetry-js/pull/4743) @pichlermarc
+ * allowing overrides of the `User-Agent` header was not specification compliant.
 
 ### :rocket: (Enhancement)