[FLINK-40549][metrics] Fix closed metricGroup leak on split finish

[Efrat19]

What is the purpose of the change

Today there's no API to deregister child metric groups added via MetricGroup#addGroup. This is not a problem as long as child metric groups are permanent through parent lifecycle.

For split level metric group this is a problem because they might not live as long as the source (aka splitFinished). As a result we are left with a closed child metric group reference on the parent, preventing the object GC.

Brief change log

• Remove metricGroup parent reference upon close()
• Close splitMetricGroup along with the child splitWatermarkMetricGroup

Verifying this change

This change added tests and can be verified as follows:

(example:)

• Unit tests ensuring the metric groups are closed and dereferenced by the parent.

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: (yes / no / don't know)
• 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 applicable

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Co-Generated-by: claude-fable-5

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CI report:

• b85b703 Azure: FAILURE

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