Add changelog

Author: DylanRussell

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - opentelemetry-sdk: Fix invalid `type: ignore` that causes mypy to ignore the whole file
   ([#4618](https://github.com/open-telemetry/opentelemetry-python/pull/4618))
+- Add `span_exporter` property back to `BatchSpanProcessor` class ([#4621](https://github.com/open-telemetry/opentelemetry-python/pull/4621)).
 
 ## Version 1.34.0/0.55b0 (2025-06-04)