Add CHANGELOG entry

Author: phillipuniverse

CHANGELOG.md

@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- `opentelemetry-sdk` - Fix the type hint of the `_metrics_data` property to allow `None` ([#4837](https://github.com/open-telemetry/opentelemetry-python/pull/4837)
+
 ## Version 1.39.0/0.60b0 (2025-12-03)
 
 - `opentelemetry-api`: Convert objects of any type other than AnyValue in attributes to string to be exportable