code: public boolean track(TopologyAPI.StreamId stream, long ts) {
      Long currentVal = streamToTs.get(stream);
      if (currentVal == null || ts > currentVal) {
        streamToTs.put(stream, ts);
      }
      return ts >= lastWaterMarkTs;
    }
docs: Tracks the timestamp of the event in the stream, returns
  true if the event can be considered for processing or
  false if its a late event.