Add log for unreferenced file cleanup (opensearch-project#11380)

Signed-off-by: RS146BIJAY <rishavsagar4b1@gmail.com>

Author: RS146BIJAY

server/src/main/java/org/opensearch/index/engine/Engine.java

@@ -1321,6 +1321,7 @@ public void failEngine(String reason, @Nullable Exception failure) {
                     // clean all unreferenced files on best effort basis created during failed merge and reset the
                     // shard state back to last Lucene Commit.
                     if (shouldCleanupUnreferencedFiles() && isMergeFailureDueToIOException(failure, reason)) {
+                        logger.info("Cleaning up unreferenced files as merge failed due to: {}", reason);
                         cleanUpUnreferencedFiles();
                     }