feat(apple): Add metrics docs

[philipphofmann]

Pre-merge checklist

If you work at Sentry, you're able to merge your own PR without review, but please don't unless there's a good reason.

• [x] Checked Vercel preview for correctness, including links
• [x] PR was reviewed and approved by any necessary SMEs
• [x] PR was reviewed and approved by a member of the Sentry docs team

Description of changes

Add metrics docs for the Cocoa/Apple SDK. We have to wait until we release Cocoa 8.23.0 to merge this.

This PR is based on https://github.com/getsentry/sentry-docs/pull/9374.

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 4, 2024 5:17am

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Bundle Report

Changes will increase total bundle size by 159 bytes :arrow_up:

Bundle name	Size	Change
sentry-docs-server	6.11MB	168 bytes :arrow_up:
sentry-docs-edge-server	618.05kB	3 bytes :arrow_down:
sentry-docs-client	5.52MB	6 bytes :arrow_down: