@Override
  public String toString() {
    return "DataNodeTracker :[" + dataNodeId.toString() + " " + activeGroupTrackers.toString() + " "
        + standByGroupTracker.toString() + "]";
  }
@return iterates over all active group trackers and returns trackers which are not tracking
any remote replica group
ambry-replication/src/main/java/com/github/ambry/replication/continuous/DataNodeTracker.java