@Override
  public Map<String, Map<String, Long>> getContainerStorageUsage() {
    return Collections.unmodifiableMap(containerStorageUsageForCurrentMonthRef.get());
  }
Constructor to instantiate a {@link MySqlStorageUsageRefresher}.
@param accountStatsStore The {@link AccountStatsStore} to interact with mysql database.
@param scheduler The {@link ScheduledExecutorService} to schedule background tasks.
@param config The {@link StorageQuotaConfig}.
@param metrics The {@link StorageQuotaServiceMetrics} to update metrics
@throws IOException
ambry-quota/src/main/java/com/github/ambry/quota/storage/MySqlStorageUsageRefresher.java