feat(api): Add a PUBLIC_EXPERIMENTAL publish status

[DominikB2014]

This Pr makes two changes:

1. Adds PUBLIC_EXPERIMENTAL to ApiPublishStatus, filling the gap between an endpoint being invisible and being frozen under a permanent stability commitment — it is published, but free to change incompatibly. Plan is to make most of our private apis have this status, so code mode and external agents immediately benefit
2. Set the /trace-items/metrics endpoint to public experiemental (the first example)

There is a complementary docs pr here getsentry/sentry-docs#19320, with a screenshot from me testing this all locally

Publication is now expressed once as ApiPublishStatus.is_published, and every gate that keyed on PUBLIC reads it (the OpenAPI filter, the S022 response-shape lint, and the Seer public-API matrix), so the new status clears the exact same documentation bar as PUBLIC rather than offering a shortcut past it. Because nothing downstream renders the x-sentry-experimental marker these operations now carry, the build also prepends a short notice to the rendered description — four endpoints were already hand-writing that warning into their docstrings in four different wordings, one of them buried at the end, and this standardizes on the most common of them. The trace-items/metrics endpoint adopts the status as the first user: publishing it pulls it into the Seer matrix, which needs its feature flag registered there, and it gains a response example, with its response TypedDicts moving to a *_types.py so the examples module can import them without a circular import. Follow-ups: a sentry-docs badge reading x-sentry-experimental to replace the text notice, the Sentaur bot tolerating the new public_experimental key in the ownership stats JSON, and migrating those four hand-rolled endpoints onto the status.

[cursor]

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[linear-code]

ENG-8593