Add span based feature flags

[adinauer]

📜 Description

Add addFeatureFlag method to span and transaction.

It'll allow setting a max of 10 feature flag evaluations per span.
Spans do not inherit or share flags, so each span is separate.
When the limit has been reached new values are rejected but existing values may be updated.

💡 Motivation and Context

Another part of #4763

💚 How did you test it?

📝 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

[github-actions]

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

Generated by 🚫 dangerJS against 39dbf57

[cursor]

Bug: Feature Flag Data Handling Causes Unintended Mutations

The feature flag logic in the SentrySpan constructor directly modifies the data map parameter. This results in feature flags being silently dropped if data is null, and it mutates the original Span's data map if data is not null, which can cause unintended side effects.

 

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	367.76 ms	460.02 ms	92.26 ms
Size	1.58 MiB	2.12 MiB	549.76 KiB

Previous results on branch: feat/feature-flags-on-spans

Startup times

Revision	Plain	With Sentry	Diff
8a5f7d1	322.31 ms	431.93 ms	109.63 ms
cb9cf5e	313.21 ms	359.20 ms	46.00 ms

App size

Revision	Plain	With Sentry	Diff
8a5f7d1	1.58 MiB	2.12 MiB	549.68 KiB
cb9cf5e	1.58 MiB	2.12 MiB	549.76 KiB