[FLINK-37410][runtime/metrics] Split level Watermark metrics

[Efrat19]

What is the purpose of the change

This pull request adds split level watermark metrics, covering watermark progress and per-split state gauges (active, idle and paused)
The change is widely described in FLIP-513: Split-level Watermark Metrics

Brief change log

• A new metric group InternalSourceSplitMetricGroup introduced as a sub-group of OperatorMetricGroup, containing a watermark reference and pauseable timers for states.
• The MG is initialized per split by SourceOperator, which already keeps track of splits pause/resume splits.
• TimestampsAndWatermarks.WatermarkUpdateListener and WatermarkOutputMultiplexer.WatermarkUpdateListener interfaces were extended to bubble split-level idleness updates, upstream to SourceOperator. (so it can switch the split idle timer on and off as well)
• WatermarkUpdateListener usages were updated to match the new signature.

Verifying this change

This change added tests and can be verified as follows:

• The new metric group, as well as state transitions reporting under alignment / idleness are unit tested.
• The change was manually verified by running a flink job reading from 2 sources, fast and slow, and verifying split level metrics reports were aligned with the watermark, paused and idle status of each split.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): yes (metrics)
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs

[flinkbot]

CI report:

• 30a3a20 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[Efrat19]

@flinkbot run azure

[davidradl]

nit:this split is active -> this split has been active

[davidradl]

nit: since registered -> since it was registered.
is this updated every second? If so then we should say that

same for the next 2 , it was and full stop

[Efrat19]

@flinkbot run azure