[FLINK-21766][metrics] Remove OperatorMetricGroup#parent()

[zentol]

Removes OperatorMetricGroup#parent() to prevent runtime code from relying on the group parent having a specific type.

Instead it now directly exposes the TaskIOMG, and the JobMG as a plain MetricGroup.

Later on we can remove the OperatorMG-internal reliance on the parent by explicitly passing in these objects.

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[tillrohrmann]

Thanks for creating this PR @zentol. LGTM. +1 for merging.