[FLINK-40648][metrics] Do not report a metric whose variables collide into one label name

[MartijnVisser]

What is the purpose of the change

Two metric group variables that differ only in characters we replace become one label name. Prometheus then rejects the whole scrape, so one such metric costs us every other metric of that process.

Brief change log

• Stop reporting a metric whose variables collide into one label name
• Warn once per colliding label name

Verifying this change

This change added tests and can be verified as follows:

• metricIsNotReportedWhenTwoVariablesSanitiseToTheSameLabelName fails before this change and passes after it

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, AbstractPrometheusReporter is @PublicEvolving. No signature changes, but a colliding metric that used to be reported no longer is
• 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 applicable

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

• Yes (please specify the tool below)

Generated-by: Claude Code (Claude Opus 5)

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CI report:

• cd40837 Azure: FAILURE

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