public static String statsPartitionKey(int partitionId) {
    // This is the same value as AmbryPartition.toString. However, in AmbryPartition, the partition id is long type.
    // Here it's an integer.
    return "Partition[" + partitionId + "]";
  }
Return partition key for stats report. For now, it's Partition[partitionId].
@param partitionId the partition id.
@return The partition key for stats report.
ambry-utils/src/main/java/com/github/ambry/utils/Utils.java