private int getLiveThreads(List<ReplicaThread> replicaThreads) {
    int count = 0;
    for (ReplicaThread thread : replicaThreads) {
      if (thread.isThreadUp()) {
        count++;
      }
    }
    return count;
  }
Register metrics for measuring the number of active replica threads.

@param replicaThreads A list of {@link ReplicaThread}s handling replication.
@param datacenter The datacenter of the {@link ReplicaThread} is running
ambry-replication/src/main/java/com/github/ambry/replication/ReplicationMetrics.java