Support span level measurements

### Context
According to our develop docs the [span protocol supports measurements](https://develop.sentry.dev/sdk/event-payloads/span/).
The [relay protocol definition includes measurements](https://github.com/getsentry/relay/blob/f51ca9435f986e104e245dbd48eab880e03662e7/relay-event-schema/src/protocol/span.rs#L82) 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.

Also see https://github.com/getsentry/team-mobile/issues/172.

Consider changing sending frames delay data to span level measurements when achieving this.