Support span level measurements

[krystofwoldrich]

Description

• Meta issue https://github.com/getsentry/team-mobile/issues/172

According to our develop docs the span protocol supports measurements. The relay protocol definition includes measurements as well.

Up until now, our SDKs set the measurement value on the root transaction instead of the span itself. We should change that to allow measurements on a span level. This will help us reporting slow + frozen frames, frame delay, etc. going forward to span streaming.

### Blocked by
- [ ] https://github.com/getsentry/sentry-javascript/issues/10398

### Tasks
- [ ] Add slow and frozen frame measurements to spans
- [ ] Add frame delay to spans (Add frame delay for RN is a separate issue)

┆Issue is synchronized with this Jira Improvement by Unito

[maxkosty]

This has been requested by a prospective enterprise customer (see linked Jira)

[maxkosty]

Same customer also hoping to clarify that this ticket is tracking automatic instrumentation of frame drops and stalls, not just an intermediate step toward that goal requiring manual instrumentation.

[krystofwoldrich]

automatic instrumentation of frame drops and stalls

This is automatically tracked for Transaction at the moment and when Span level measurements this frame drops and stalls will be added to spans when span level measurements are implemented.

This doesn't include any new auto instrumented span. So if the customer doesn't have any span/transaction in the part of the app they are interested this feature won't change the current state.