@Override
  public void shutdown() {
    if (scheduler != null) {
      Utils.shutDownExecutorService(scheduler, config.refresherPollingIntervalMs, TimeUnit.MILLISECONDS);
    }
  }
This {@link QuotaEnforcer} doesn't require any {@link QuotaSource} since it will fetch the quota internally.
@return a null QuotaSource.
ambry-quota/src/main/java/com/github/ambry/quota/storage/StorageQuotaEnforcer.java