You can post RFE for this. It should be possible to manually instrument app via OneAgend SDK but this is something that needs experiments.
I doubt that will work without any additional modules. I assume there needs to be a bridge between the flutter code and the native api. (But @Radu L. knows probably more about this.)
Is this possible? If so where is there documentation on how to do it?
No new information about this. You should create product idea (if there wasn't any before).
There is now a Flutter plugin available:
Instrument mobile app with Dynatrace Flutter plugin