feat(metrics): Add public API to collect count, distribution and gauge

[philprime]

📜 Description

This pull request introduces a new metrics API to the codebase, allowing clients to record custom metrics such as counts, distributions, and gauges. The API is exposed via the SentrySDK.metrics property and is designed for future extensibility, though the current implementation is a no-op.

This PR also adds an example UI to manually collect metrics in iOS-Swift.

💡 Motivation and Context

Closes #6949

💚 How did you test it?

• Added unit tests

📝 Checklist

You have to check all boxes before merging:

• I added tests to verify the 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.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

[github-actions]

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

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[codecov]

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