Support span level measurements

[krystofwoldrich]

• Meta issue [EPIC] Support span level measurements team-mobile#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.

• Related RN issue Support span level measurements  sentry-react-native#3566

[mydea]

We should implement this as span events (to be otel compatible). We need this for v8 to replace the current measurements that are tied to transactions (which are going away)!

[AbhiPrasad]

This will be done with #10945

[AbhiPrasad]

This is no longer the case because we'll support numeric attributes with EAP span.setAttribute(KEY, 123);