implement multivalue_doc_count aggregation

Signed-off-by: 김민수 <35398494+chamchi00800@users.noreply.github.com>

Author: chamchi00800

CHANGELOG.md

@@ -19,6 +19,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add tracing support for StreamingRestChannel  ([#20361](https://github.com/opensearch-project/OpenSearch/pull/20361))
 - Introduce new libs/netty4 module to share common implementation between netty-based plugins and modules (transport-netty4, transport-reactor-netty4) ([#20447](https://github.com/opensearch-project/OpenSearch/pull/20447))
 - Add validation to make crypto store settings immutable ([#20123](https://github.com/opensearch-project/OpenSearch/pull/20123))
+- Add multivalue_doc_count aggregation for counting documents with multivalued fields ([#20329](https://github.com/opensearch-project/OpenSearch/issues/20329))
 
 ### Changed
 - Handle custom metadata files in subdirectory-store ([#20157](https://github.com/opensearch-project/OpenSearch/pull/20157))
@@ -88,4 +89,4 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 - Remove identity-shiro from plugins folder ([#20305](https://github.com/opensearch-project/OpenSearch/pull/20305))
 
-[Unreleased 3.x]: https://github.com/opensearch-project/OpenSearch/compare/3.4...main
+[Unreleased 3.x]: https://github.com/opensearch-project/OpenSearch/compare/3.4...main