docs(apple): Document Set as a supported metric attribute type (#17671)

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## DESCRIBE YOUR PR

Add Set<String>, Set<Bool>, Set<Int>, Set<Double>, and Set<Float> to the
list of supported SentryAttributeValue types in the Apple metrics docs,
following the addition of Set conformance in sentry-cocoa#7876.

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## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>