Fix the type hint of _metrics_data within InMemoryMetricReader (#4837)

* Fix the type hint of _metrics_data within InMemoryMetricReader

This gets initialized to None, so the type hint should be MetricsData | None

* Add CHANGELOG entry

* Update CHANGELOG.md

---------

Co-authored-by: Riccardo Magliocchetti <[email protected]>

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)
 - Regenerate opentelemetry-proto code with v1.9.0 release
   ([#4840](https://github.com/open-telemetry/opentelemetry-python/pull/4840))
 - Add python 3.14 support

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/export/__init__.py

@@ -406,7 +406,7 @@ def __init__(
             preferred_aggregation=preferred_aggregation,
         )
         self._lock = RLock()
-        self._metrics_data: MetricsData = None
+        self._metrics_data: MetricsData | None = None
 
     def get_metrics_data(
         self,