Overwrite TTFD measurement when API called too early

[stefanosiano]

Description

If the API Sentry.reportFullyDisplayed is called too early, we overwrite the TTFD measurement, but we set it in the TTID span, not the TTFD one.
The problem is that the TTFD span is already finished, and we can't set measurements to already finished spans

[markushi]

Let's allow changing the span/txn values via a private APIs.

[kahest]

@markushi @stefanosiano is this still relevant?

[markushi]

@markushi @stefanosiano is this still relevant?

Yes!

[stefanosiano]

We can add a private flag in the integration that is set when the user calls the API.
If the ttid is already finished, then all fine. If not, whenever the ttid finishes, the SDK finishes the ttfd, too.
This would fix everything 💯