Rename dag serialization metrics.

[Ei-Sandi]

Description

Follow-up to #68906, which added a single dag.serialization_writes counter emitted from two different branches of SerializedDagModel.write_dag. Sharing one name means the metric can't distinguish the two ways a Dag gets serialized, and they have very different consequences:

• a new DagVersion row is inserted — unbounded growth across dag_version, dag_code and serialized_dag
• the latest DagVersion is overwritten in place — bounded churn on a single row

A Dag that re-serializes on every parse (for example one whose template holds a callable, with no actual change to the Dag) shows up in either path depending on whether its current version has task instances, but only the first grows the metadata DB. Splitting the counter lets the two be alerted on with different thresholds.

dag.serialization_writes is replaced by:

metric	emitted when
dag.serialization.version_created	a new Dag version is written — first serialization, or the latest version already has task instances
dag.serialization.version_updated	the latest Dag version is overwritten in place, having no task instances

The metric has not been released yet, so there is no deprecation path and no newsfragment.

The paths that write no serialized data — a bundle-metadata refresh, or a parse where nothing changed — are still uncounted.

related: #68906

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Co-authored-by: [Claude Opus 5] following the guidelines

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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.
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[SameerMesiah97]

Looks good.

I think this does need a newsfragment as you are adding a new metric. At least, that was what a maintainer asked for when reviewing 2 recent PRs of mine that were adding new metrics to the Triggerer.