Add metrics for how often a dag gets serialised.

[Ei-Sandi]

Changes

• Adds a new dag.serialization counter metric that is emitted every time a Dag is serialized and written to the metadata DB.
• The metric is tagged with dag_id and bundle_name, so serialization frequency can be tracked per Dag and per bundle.

(This gives operators visibility into how often DAGs are being re-serialized, to ensure we aren't unnecessarily re-serializing DAGs. )

Tests

• Covered by unit tests.

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

• Yes (please specify the tool below)
  Co-authored-by: Claude Sonnet 4.6, Claude Opus 4.8 following the guidelines

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[xBis7]

Mostly looks good! I have a few suggestions for the tests. Thanks!

BTW, I've tested it manually with breeze and metrics work as expected.

[Ei-Sandi]

@xBis7 The comments has been addressed. Can you please review it again ? Thank you.