Adding the change in CHANGELOG.md

Signed-off-by: Kshitij Tandon <tandonks@amazon.com>

Author: tandonks

CHANGELOG.md

@@ -22,6 +22,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Introduce system generated ingest pipeline ([#17817](https://github.com/opensearch-project/OpenSearch/pull/17817)))
 - Support create mode in pull-based ingestion and add retries for transient failures ([#18250](https://github.com/opensearch-project/OpenSearch/pull/18250)))
 - Decouple the init of Crypto Plugin and KeyProvider in CryptoRegistry ([18270](https://github.com/opensearch-project/OpenSearch/pull18270)))
+- Supporting Scripted Metric Aggregation when reducing aggregations in InternalValueCount and InternalAvg ([18288](https://github.com/opensearch-project/OpenSearch/pull18270)))
 
 ### Changed