[EPIC] Support span level measurements

[markushi]

Context

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.

Tasks

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1. Support span level measurements sentry-javascript#10398
   Feature: Spans +1
2. Support span level measurements sentry-react-native#3566
   0 of 3
   Blocked Platform: React-Native Sync: Jira +3
3. Support span level measurements sentry-cocoa#3599
   Platform: Cocoa +1
4. Support span level measurements sentry-java#3180
   Platform: Android Platform: Java +2
   
5. Support span level measurements sentry-dart#1855
   Platform: Dart +1
   
6. Support span level measurements sentry-capacitor#562
   0 of 1
   Platform: Capacitor Status: Backlog +2

[markushi]

I just briefly talked to @AbhiPrasad: This should definitely be doable, but some ingestion related topics (sampling, scrubbing) might not be fully setup by now.

Kinda related PR: https://github.com/getsentry/relay/pull/2969/files