feat(ios): Introduce enableMetricKit option

[alwx]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

Adds enableMetricKit to the React Native SDK options, forwarding it to the Cocoa SDK.

The option name matches the Cocoa SentryOptions property, so it is decoded by SentryOptionsInternal initWithDict: and needs no mapping in RNSentryStart.m — same as enableAppHangTracking / appHangTimeoutInterval. No production code changes, only the type declaration plus tests.

Sentry.init({
  dsn: '___DSN___',
  enableMetricKit: true,
});

Notes:

• No JS default, so the key is absent from the options dict unless set and native/sentry.options.json init can still own it.
• Android ignores it (RNSentryStart.java is per-key hasKey() opt-in).
• No API report diff — BaseReactNativeOptions isn't exported from the entry point.

💡 Motivation and Context

enableMetricKit could only be enabled through native initialization. This makes it officially configurable from RN.

MetricKit hang diagnostics (MXHangDiagnostic) are reported by the OS and are distinct from App Hangs V2, so this is not a substitute for enableAppHangTracking — enabling both can double-report a hang. Documented in the JSDoc.

Fixes #6457

💚 How did you test it?

• packages/core/test/wrapper.test.ts — option is forwarded when set, absent when unset.
• RNSentryTests.m — enabled / disabled / default cases asserting SentryOptions.enableMetricKit. This is what verifies Cocoa's dict decoder actually maps the key.
• yarn build, yarn test (2073 passing), yarn lint, yarn circularDepCheck, yarn api-report (no diff).
• xcodebuild test -configuration Release on iPhone 17 / iOS 26.1 — 49 tests, 0 failures.

Not verified end-to-end against Sentry: iOS delivers MetricKit payloads on a ~24h cadence, so that needs a TestFlight build.

📝 Checklist

• I added tests to verify changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• All tests passing.
• Public API changes reviewed by another Mobile SDK team member or implemented according to the develop docs spec.
• No breaking changes.

🔮 Next steps

• Docs PR in getsentry/sentry-docs for the new option.
• enableMetricKitRawPayload is intentionally out of scope; still native-init only.

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

📋 Changelog Preview

This is how your changes will appear in the changelog.
Entries from this PR are highlighted with a left border (blockquote style).

---

• feat(ios): Introduce enableMetricKit option by alwx in #6540

---

_{🤖 This preview updates automatically when you update the PR.}

[antonis]

LGTM once the full test suite is 🟢 and the changelog conflict resolved ✅
Let's also enhance the docs 🙇

[github-actions]

	Fails
🚫	Pull request is not ready for merge, please add the "ready-to-merge" label to the pull request

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 1a1187c

[alwx]

@antonis did the docs PR: getsentry/sentry-docs#18881

[alwx]

Fixed.