feat(flags): capture feature flag evaluations on spans

[aliu39]

Based off https://github.com/getsentry/sentry-python/pull/4280/files
Reopened from #16475

This PR updates our 5 browser FF integrations to capture flag evaluations on spans. This implementation avoids changes to the Span class, saving bundle size.

• on eval, flags are added to span attributes. A global WeakMap is used to track the unique flags for a span. Updates to a flag's value is allowed.
• staying consistent with the python PR:
  • we only capture values for the first 10 unique flags per span. Subsequent flags are dropped.
  • attribute keys have the format flag.evaluation.{flagKey}

From @AbhiPrasad :

A method on the span break our compat with OTEL spans, which we use for duck typing in some scenarios.

From @cmanallen :

For spans the oldest flag evaluations are the most important because they're likely to have the largest performance impact. We drop late arrivals due to request size concerns.

[aliu39]

Useful refs:

• https://develop.sentry.dev/sdk/platform-specifics/javascript-sdks/browser-tracing/
• https://develop.sentry.dev/sdk/data-model/event-payloads/span/

[chargome]

Looks good! Just one question: You need the buffer approach for determining the evaluation order right? Otherwise we could also just conditionally add the flag based on how many attributes are already present on the span.

[aliu39]

Looks good! Just one question: You need the buffer approach for determining the evaluation order right?

Yes, I originally used buffers/arrays to track the eval order as we do in the scope/errors. However I don't believe this is a requirement as we're not enforcing order in Python spans, only recording the first 10 unique flags. IMO it's not as valuable for the span usecase, and for only 10 flags.

Otherwise we could also just conditionally add the flag based on how many attributes are already present on the attributes.

Yes that would also work as an alternative to the global weak map. Is there a way to do this without extending the Span interface though?

[chargome]

@aliu39 something like

if (shouldAddFeatureFlagToSpan(Sentry.spanToJSON(span).data, someFlag)) {
  span.setAttribute(
    // ...
  )
}

And in shouldAddFeatureFlagToSpan you can count the attributes starting with your prefix for example.

[chargome]

Don't want to block you on this – we can also refactor this part in a follow up!

[billyvg]

Is there somewhere better to save this map, rather than globally?

[aliu39]

Not sure - maybe the client? We can follow up, also with Charlie's proposal we wouldn't need the map anymore.