@Override
  public String toString() {
    return "StandByGroupTracker :[" + super.toString() + "]";
  }
This class tracks for a current state for StandBy groups for a continuous replication cycle.
Standby group has  replicas with STANDBY_TIMED_OUT state only
ambry-replication/src/main/java/com/github/ambry/replication/continuous/StandByGroupTracker.java