[lmilcin]

Why not write a simple service that tracks various stats (like number of users, requests, etc.) as well as billed costs over time?

You could then get various interesting stats in real time as well as some pretty useful alerting.

[nrmitchi]

Even with "infinite scale", you should still be monitoring, and be doing some form of budget monitoring.

The difference is that application performance metrics are generally available in near-real time, whereas billing metrics are 1) very platform specific, and 2) generally not even close to real time.

It's hard to react quickly when your platform has effectively transformed near-real-time performance alerts to delayed/rolled-up billing alerts (which would also be much more difficult to use to pinpoint where the underlying issue is)