code: @Override
      public TriggerResult onElement(Object element, long timestamp, TimeWindow window, TriggerContext ctx) {
          ctx.registerEventTimeTimer(window.getEnd() + ctx.getMaxLagMs(), window);
          return TriggerResult.CONTINUE;
      }
docs: If a watermark arrives, we need to check all pending windows.
  If any of the pending window suffices, we should fire immediately by registering a timer without delay.
  Otherwise we register a timer whose time is the window end plus max lag time