@Override
    public void onClusterRebalanced(final ClusterStatus clusterStatus) {
        timeOfLatestStabilitySts = TimeBucket.getMinuteTimeBucket(
            clusterStatus.getRebalancedTime());
    }
Check the metrics whether in the cache, and whether the worker should go further to load from database.

@param metrics the metrics in the streaming process.
@return metrics in cache or null if try to read the metrics from the database.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/worker/MetricsPersistentWorker.java