How to count external traffic in CDN/farm?

Question

Web traffic analysis

Good afternoon,
So, what will be rather not a CDN, but simply a server farm like a file sharing service (without a local network).
Each large file is cut into a bunch of small ones from 2mb to 12mb and has an id that is not associated with the user, but belongs to him.
It is necessary to count the outgoing traffic of all files belonging to a particular user.
I can write everything myself (it is possible to write scripts for the web server).
We need an idea how to make it so that this does not affect performance (file upload) + statistics are available to the user almost online by clicking.
PS I do not want to install additional server modules and software. It is possible to write a server part. So we need an idea.