This is mostly a "feature request" for Google.
Germanys biggest carrier (Deutsche Telekom) allows apps to be zero rated (meaning their usage doesn't consume any of your plans mobile data). According to them it is completely free for the apps' publisher and just requires a registration.
More information for users can be found on their page (in German): https://www.telekom.de/hilfe/mobilfunk-mobiles-internet/mobiles-internet-e-mail/streamon/was-ist-str...
Since we just got true mobile gaming on Stadia, can Google please register Stadia with StreamOn, so it's traffic won't be counted?
Other carriers offer such options as well, so having Google talk with Carriers to provide endless mobile game streaming would clearly be a massive benefit for Stadia users.