It's not that hard. If you're already using a module loader (which in all likelihood, you are if you're loading something async these days), then just count modules being loaded in your controller(s), and use that as your basis.

A harder approach would be to be super accurate with actual size of response. Content-length on the header won't work because it only refers to that one file. Also, the server won't have any clue which files belong to which web page response. A way around this is to check every request against a session ID. You'll also have to pass back some sort of "request hash" -- GUID for the request so you can match different files from the same request (static files, content, etc).