Add entry in changelog

Author: fcollonval

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Fix metrics export with exemplar and no context and filtering observable instruments
+  ([#4251](https://github.com/open-telemetry/opentelemetry-python/pull/4251))
+
 ## Version 1.28.0/0.49b0 (2024-11-05)
 
 - Removed superfluous py.typed markers and added them where they were missing