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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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[Lee-W]

let's rebase from main and resolve the conflicts. thanks!

[FrankYang0529]

let's rebase from main and resolve the conflicts. thanks!

Resolved it. Thanks.

[henry3260]

Overall. looks good just 2 nits

[github-actions]

Backport failed to create: v3-3-test. View the failure log Run details

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
❌	v3-3-test

You can attempt to backport this manually by running:

cherry_picker f44634c v3-3-test

This should apply the commit to the v3-3-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

If you don't have cherry-picker installed, see the installation guide.