@Override
  public MetricRegistry getMetricRegistry() {
    return helixClusterManager.getMetricRegistry();
  }
The list of {@link DataNodeId}s present in the underlying {@link StaticClusterManager}
Also verify that the underlying {@link HelixClusterManager} has the same set of nodes, if not, update a metric.
@return the list of {@link DataNodeId}s present in the underlying {@link StaticClusterManager}
ambry-clustermap/src/main/java/com/github/ambry/clustermap/CompositeClusterManager.java