Since the discussion about the change proposal triggered so many replies (some of those implying malevolence in moving the discussion from ML to Discourse and split in separate threads), let’s get down to the core question and make use of the advanced feature of Discourse to have users clearly state...
It's off by default (opt-in). To increase visibility of this option, it's OK to ask the user via popup, during setup, or similar, whether he would like to turn it on
Once it's on, the user should be informed BEFORE each upload takes place about the exact content which should be uploaded, and asked whether he wants to send it (Y/N)
Transmission must only contain truly necessary data
I agree with everything, except the second point. If I already opted-in I don't want to be prompted every time that telemetry is uploaded. I wouldn't mind a notification informing me that telemetry has been sent while also allowing me to review it.
Also, it almost goes without saying that I should also be able to opt-out at any time, even after explicitly opting-in.