public static String getPartitionLagKey(PartitionId partitionId, String hostname, String replicaPath) {
    return partitionId.toString() + ":" + hostname + ":" + replicaPath;
  }
Gets the key for the lag override in {@code lagOverrides} using the given parameters.
@param partitionId the {@link PartitionId} whose replica {@code hostname} is.
@param hostname the hostname of the replica whose lag override key is required.
@param replicaPath the replica path of the replica whose lag override key is required.
@return
ambry-replication/src/test/java/com/github/ambry/replication/MockReplicationManager.java