Update CHANGELOG.md for mutex refactoring improvements

Signed-off-by: Sean Chittenden <sean.chittenden@crowdstrike.com>

Author: sean-

CHANGELOG.md

@@ -7,6 +7,8 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Added
 
 ### Changed
+- Refactor Client struct to use embedded mutex pattern for improved thread safety ([#775](https://github.com/opensearch-project/opensearch-go/pull/775))
+- Refactor metrics struct to use atomic counters for lock-free request/failure tracking ([#776](https://github.com/opensearch-project/opensearch-go/pull/776))
 
 ### Deprecated