void trackLiveThreadsCount(final List<ReplicaThread> replicaThreads, String datacenter) {
    Gauge<Integer> liveThreadsPerDatacenter = () -> getLiveThreads(replicaThreads);

    registry.gauge(MetricRegistry.name(ReplicaThread.class, "NumberOfReplicaThreadsIn" + datacenter),
        () -> liveThreadsPerDatacenter);
  }
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