Detect stale metrics registry entries in the metrics sync prek hook

[FrankYang0529]

Why

• check-metrics-synced-with-registry only checks one direction: a metric used in code but missing from metrics_template.yaml.
• A registry entry no longer emitted anywhere was undetected.

How

• The check scans all git-tracked non-test Python files (git ls-files). It checks any registry entry with no matching metric in code.
• Add an INDIRECTLY_EMITTED_METRICS allowlist for the three executor.* gauges whose names are built via BaseExecutor._get_metric_name() and cannot be resolved statically.

• related: Remove stale metrics no longer emitted from the metrics registry #68945

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

• Yes - Claude Code

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