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

[philprime]

:scroll: 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.

:bulb: Motivation and Context

Closes #6949

:green_heart: How did you test it?

• Added unit tests

:pencil: Checklist

You have to check all boxes before merging:

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

[github-actions[bot]]

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

Generated by :no_entry_sign: dangerJS against 2cd3b89b3531cdee8770a3f3d1d7a174d7929a4d

[codecov[bot]]

:warning: JUnit XML file not found

The CLI was unable to find any JUnit XML files to upload. For more help, visit our troubleshooting guide.

[philprime]

@sentry review

[github-actions[bot]]

Performance metrics :rocket:

	Plain	With Sentry	Diff
Startup time	1209.52 ms	1247.46 ms	37.94 ms
Size	24.14 KiB	1.05 MiB	1.02 MiB

Baseline results on branch: philprime/metrics-envelope

Startup times

Revision	Plain	With Sentry	Diff
63535b82c02e659862de227590d66775f51b534a	1228.64 ms	1255.27 ms	26.63 ms
34e473acf0c2231f4c7aecda08469c51f6b8054e	1195.53 ms	1222.53 ms	27.00 ms
f1ee7f4a4a468d79e01d7f9f3841307824f95636	1221.29 ms	1248.16 ms	26.87 ms

App size

Revision	Plain	With Sentry	Diff
63535b82c02e659862de227590d66775f51b534a	24.14 KiB	1.04 MiB	1.01 MiB
34e473acf0c2231f4c7aecda08469c51f6b8054e	24.14 KiB	1.04 MiB	1.01 MiB
f1ee7f4a4a468d79e01d7f9f3841307824f95636	24.14 KiB	1.04 MiB	1.01 MiB

Previous results on branch: philprime/metrics-public-api

Startup times

Revision	Plain	With Sentry	Diff
a35c105f6b0455c37eb2906b0939e4b54b0f4852	1220.52 ms	1247.74 ms	27.22 ms
a7831e5affca02e149d7a4f3849a625cd9b6e31d	1210.38 ms	1244.09 ms	33.71 ms

App size

Revision	Plain	With Sentry	Diff
a35c105f6b0455c37eb2906b0939e4b54b0f4852	24.14 KiB	1.05 MiB	1.02 MiB
a7831e5affca02e149d7a4f3849a625cd9b6e31d	24.14 KiB	1.05 MiB	1.02 MiB

[philprime]

Converted back to draft until I properly handle the typing of the integrations

[philprime]

@sentry review @cursor review