Add log for unreferenced file cleanup (#11380)

Signed-off-by: RS146BIJAY <rishavsagar4b1@gmail.com>
(cherry picked from commit 4f7b2a4)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

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

@@ -1373,6 +1373,7 @@ 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();
                     }