The way you describe requires somehow counting every bit but somehow discarding most which is obviously nonsense.
This seems statistically invalid insofar as it will tend to overbill potentially by a lot on the minority of cases.
Don't you know how much of the pipe is occupied by a given customers code at any given time or what data is being sent
You have to do it when the customer list is too big to keep a counter per customer.
A probabilistic counter per customer is also a counter per customer. Still, probabilistic billing is an amusing thought though.
No you don't