title: How to retrieve and customize Watermark in Dataflow?
body: <p>I'm not sure if this is considered best practices or not, but I'm trying to figure out a way to customize the watermark for Dataflow. Similar to Flink's <code>assignTimestampsAndWatermarks()</code>.</p>
  
  <p>I'm aware of <code>timestampBy()</code>, I want to be able to able to retrieve the current watermark and apply some additional logic that is dependent on it. But I can't seem to find any way to retrieve the current watermark.</p>