feat(envelope-item): Support metric type

[lucas-zimerman]

📜 Description

Metric support - this PR is needed for Capacitor/Dart/Flutter/React-Native right now but will be needed in general in the future when Java also implements Metric support

💡 Motivation and Context

See metric protocol: https://develop.sentry.dev/sdk/telemetry/metrics/

Needed for processing envelope items with type trace_metric. Without this change, Hybrid SDKs will lose their metric data by replacing type trace_metric by __unknown__.

💚 How did you test it?

Unit test

📝 Checklist

• I added GH Issue ID & Linear ID
• 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.

🔮 Next steps

Close #4957

[lucas-zimerman]

failing test is due to me not having permission on this repo.

[antonis]

LGTM 🚀

Also tested locally and confirmed that metrics are displayed for Android.

[romtsn]

LGTM!

[lucas-zimerman]

working as expected 🗡️

[lucas-zimerman]

just adding a note that I can't merge PRs here :D