POC startSpan API

[adinauer]

📜 Description

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• 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.
• 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

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

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

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- POC `startSpan` API ([#3724](https://github.com/getsentry/sentry-java/pull/3724))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

Generated by 🚫 dangerJS against abb5bf6

[adinauer]

FYI @markushi just so you're aware this exists, looks like we won't be using this atm though.

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	471.69 ms	581.84 ms	110.15 ms
Size	1.70 MiB	2.35 MiB	667.29 KiB