public static String partitionClassStatsAccountContainerKey(short accountId, short containerId) {
    return "A[" + accountId + "]" + ACCOUNT_CONTAINER_SEPARATOR + "C[" + containerId + "]";
  }
Return PartitionClassReport stats account container entry key of given {@code accountId} and {@code containerId}.
For now, it's A[accountId]___C[containerId].
@param accountId the account id.
@param containerId the container id.
@return the account container entry key in PartitionClassReport stats.
ambry-utils/src/main/java/com/github/ambry/utils/Utils.java