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.

[xBis7]

LGTM!

[Miretpl]

As far as I know this part of the codebase - looks good. I will wait a couple of days with the merge for any other possible reviews.

[vatsrahul1001]

This looks more like a new feature and more suitable for minor 3.4.0

[SameerMesiah97]

Just one tiny suggestion.

[potiuk]

Re-reviewed this before merging and want one thing resolved first — the metric is sound but its two emission sites don't mean the same thing.

Things I checked and am happy with:

• The two stats.incr calls sit before mutually exclusive return True paths in write_dag, so there's no double-counting, and the early return False (min_update_interval) correctly doesn't emit.
• name_variables: ["dag_id", "bundle_name"] with a matching legacy_name accurately describes what's emitted. Worth calling out because #69078 declares name_variables: [] while tagging with two variables — yours is the correct pattern.

The nit is inline. Not a big change, but I'd rather it went in right than be corrected after people start alerting on it.

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Drafted-by: Claude Code (Opus 5); reviewed by @potiuk before posting