Watermark metrics for an operator&task shadow each other

In FLINK-4812 we reworked the watermark metrics to be exposed for each operator.
In FLINK-9467 we made further modifications to also expose these metrics again for tasks.

In both JIRAs we register a single metric multiple times, for example the input watermark metric is registered for both the first operator in the task, and the task itself.

Unfortunately, the metric system assumes metric objects to be unique, as can be seen in virtually all reporter implementations as well as the MetricQueryService.

As a result the watermark metrics override each other in the reporter, causing only one to be reported, whichever was registered last.