Add CHANGELOG

Author: xrmx

CHANGELOG.md

@@ -22,6 +22,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - `opentelemetry-instrumentation-httpx`: instrument_client is a static method again
   ([#3003](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3003))
+- `opentelemetry-instrumentation-system_metrics`: fix callbacks reading wrong config
+  ([#3025](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3025))
 
 ### Breaking changes