@Override
  public QuotaSource getQuotaSource() {
    return quotaSource;
  }
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