Github user zentol commented on the issue:
https://github.com/apache/flink/pull/5100
the coupling of input to output gauge becomes rather tricky. The input
gauge must be reachable in `Stream[Two]InputProcessor`,
`OperatorChain#createChainedOperator`, and the output gauge in
`AbstractStreamOperator#setup`.
For sources, sinks and watermark assigners we must refer to the same gauge
in both places. I don't see how we can do this in a nice way.
The rather nice thing the current approach has is that everything is
contained in `AbstractStreamOperator` and isn't scattered all over the place.
`Given that we allow people to completely re-define how operators handle
watermarks, wouldn't it make sense to give them the same freedom when it comes
to the input watermark metrics?
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