[FLINK-21000][runtime] Add an internal split enumerator metric group

[ruanhang1993]

What is the purpose of the change

This pull request adds an internal split enumerator metric group.

Brief change log

• Add InternalOperatorEnumeratorMetricGroup
• make SourceCoordinatorContext#metricGroup return an instance of InternalOperatorEnumeratorMetricGroup
• Define the metric name 'unassignedSplits' in MetricNames

Verifying this change

This change added unit tests.

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): no
• 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? no
• If yes, how is the feature documented? not documented

[ruanhang1993]

Hi, @zentol , this is the metric group for the SplitEnumerator. Please take a look if you have time. Thanks~

[flinkbot]

CI report:

• a0f1626 Azure: SUCCESS

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[stevenzwu]

I know it implemented by the other PR and is outside the scope of this PR. curious about both coordinator and enumerator. kind of redundant to me. do we have components (other than enumerator) under coordinator?

[ruanhang1993]

SplitEnumerator is the main component in the SourceCoordinator.I think it is clearer for the relationship between them with the name enumerator. Removing it is also fine.

[leonardBang]

Not all coordinator contains enumerator component, I think keep both of them is more clear.

[leonardBang]

Thanks @ruanhang1993 for the contribution, LGTM