Add REST API metrics

[henry3260]

What

adds REST API instrumentation using Airflow's existing metrics abstraction.

Changes included:

• Added basic API metrics emission in HttpAccessLogMiddleware.
• Emit api.requests, api.request.duration, and api.request.errors for /api/v2 and /ui requests.
• Added low-cardinality metric tags: api_surface, method, route, status_code, and status_family.

related: #56034

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

• Yes (please specify the tool below)

---

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[henry3260]

cc @pierrejeambrun