Remove stale metrics no longer emitted from the metrics registry

[FrankYang0529]

Remove the three orphaned entries from metrics_template.yaml:

• dag_processing.manager_stalls — emission removed in Remove dag processor from the scheduler #45729
• dag_file_refresh_error — emission removed in Swap Dag Parsing to use the TaskSDK machinery. #44972
• collect_db_dags — emission removed in Move DagBag to SDK and make it return SDK DAG objects #53918

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[SameerMesiah97]

I searched for these metrics in main and I could find them being emitted anywhere. I do wonder why they were not removed from .yaml file? I want to make sure removing them from the registry won't impact any tooling that relies on the template rather than the emitted metrics themselves. This looks fine to me but let's wait for a maintiainer to weigh in.

[Lee-W]

make sense to me. probably also worth a quick confirm from @ashb @jedcunningham and @uranusjr (the original author of the PRs).

we can also add a prek hook to check. e.g., if we add or remove a metrics, we trigger this hook to compare the metric in the doc and in the code

[FrankYang0529]

we can also add a prek hook to check. e.g., if we add or remove a metrics, we trigger this hook to compare the metric in the doc and in the code

Created a followup PR. Thank you.

#69578