Map<String, Map<String, Long>> getContainerStorageUsageMonthlyBase() {
    return Collections.unmodifiableMap(containerStorageUsageMonthlyBase);
  }
Return {@link #containerStorageUsageMonthlyBase}. Only used in test.
@return the {@link #containerStorageUsageMonthlyBase}.
ambry-quota/src/main/java/com/github/ambry/quota/storage/MySqlStorageUsageRefresher.java