Making _cat/allocation API use indexLevelStats (opensearch-project#15292)

Signed-off-by: Harsh Garg <gkharsh@amazon.com>
Co-authored-by: Harsh Garg <gkharsh@amazon.com>

Author: 2 people

CHANGELOG.md

@@ -54,6 +54,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Reset DiscoveryNodes in all transport node actions request ([#15131](https://github.com/opensearch-project/OpenSearch/pull/15131))
 - Relax the join validation for Remote State publication ([#15471](https://github.com/opensearch-project/OpenSearch/pull/15471))
 - MultiTermQueries in keyword fields now default to `indexed` approach and gated behind cluster setting ([#15637](https://github.com/opensearch-project/OpenSearch/pull/15637))
+- Making _cat/allocation API use indexLevelStats ([#15292](https://github.com/opensearch-project/OpenSearch/pull/15292))
 
 ### Dependencies
 - Bump `netty` from 4.1.111.Final to 4.1.112.Final ([#15081](https://github.com/opensearch-project/OpenSearch/pull/15081))

server/src/main/java/org/opensearch/rest/action/cat/RestAllocationAction.java

@@ -101,6 +101,7 @@ public void processResponse(final ClusterStateResponse state) {
                 statsRequest.clear()
                     .addMetric(NodesStatsRequest.Metric.FS.metricName())
                     .indices(new CommonStatsFlags(CommonStatsFlags.Flag.Store));
+                statsRequest.indices().setIncludeIndicesStatsByLevel(true);
 
                 client.admin().cluster().nodesStats(statsRequest, new RestResponseListener<NodesStatsResponse>(channel) {
                     @Override