Project Fi provides you updates on how much data you're using, but you can only see it cumulatively over your current billing statement, not per day or per app. Signal Spy does! 

For free users, you'll be able to see all of the apps that used data today, along with the amount used, sorted alphabetically. For Pro users, you'll be able to sort by data used, see data usage over a number of days, and be able to see data usage of each app per Fi network.