public synchronized Set<MessageInfo> getMissingStoreMessages() {
      return missingStoreMessages == null ? Collections.emptySet() : new HashSet<>(missingStoreMessages);
    }
Get missing store messages in this metadata exchange.
@return set of missing store messages as a new collection.
ambry-replication/src/main/java/com/github/ambry/replication/ReplicaThread.java