Add changelog

Author: ocelotl

Diff for: CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Fix `ExponentialHistogramAggregation`
+  ([#3978](https://github.com/open-telemetry/opentelemetry-python/pull/3978))
 - Log a warning when a `LogRecord` in `sdk/log` has dropped attributes
   due to reaching limits
   ([#3946](https://github.com/open-telemetry/opentelemetry-python/pull/3946))