[tor-relays] write-history for exit relays only?

I want to learn how many bytes exit relays forwarded. I assume that the write-history that is published in a relay's extra-info document includes bytes that were relayed as part of the exit's guard and middle role? If so, is there a way to learn how many bytes were written by the relay in its exit role only?

I suspect that one could approximate this number by accounting for the probability of all exits being selected as guard, middle, and exit, but I would prefer a simpler and more reliable approach.