[FLINK-6861][metrics] Use OperatorID in metric system

[zentol]

What is the purpose of the change

This PR introduces Operator IDs to the metric system.

Brief change log

• store OperatorID in OperatorMetricGroup
  • for the batch API, this ID is equal to the JobVertexID of the task
  • modify TaskMetricGroup to store a JobVertexID instead of AbstractID (required for conversion of task ID to operator ID)
• update OperatorScopeFormat to also accept the operator ID
• update documentation

Verifying this change

This change is already covered by existing tests, such as OperatorGroupTest#testVariables.

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, Yarn/Mesos, ZooKeeper: (no)

Documentation

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

[aljoscha]

The changes look good! Is there any test that checks using all those different variables, i.e. that would check that I can use the operator id in a scope pattern?

[zentol]

@aljoscha I've added a test. see OperatorGroupTest#testGenerateScopeCustom.

[aljoscha]

Thanks! This looks good now.