Project Fi subscribers are getting a new tool to track data usage today. A post in the Fi sub-Reddit by the verified Google community manager account says data tracking will now update live, and that usage will be broken out by app. It's all live right now—just open the app and check it out.
Data updating left to right.
This feature requires Android 7.0 or higher, but that shouldn't be a problem considering Fi is mostly limited to devices that Google updates. You need to be on Fi app version K.2.7.20 or later, but that's over a month old. You can see that version, and more recent ones on APK Mirror. This indicates the new data tracking features are probably a server-side switch.
The live updating usage will be available in the app's main screen as well as on the data usage widget. This change means that you'll be notified immediately if you exceed your data cap. The per-app usage breakdown is under "View details" for the current billing cycle.