[metrics-team] Where do the by-transport counts come from in userstats-bridge-combined?

[David Fifield]

I am referring to https://bugs.torproject.org/19544:
> What we could also do as first approximation is find a lower and upper
> bound of users by country and transport. The lower bound would
> probably be defined as something like max(0, PT + CC - 1) (not just 0
> to account for cases where CC > 1 - PT) and the upper bound as min(PT,
> CC), even though I could be convinced that other formulas are even
> more correct.

I thought I understood this but I guess I do not. Does PT come from
dirreq-v3-reqs and CC from bridge-ip-transports? That wouldn't make
sense to me, because they are measuring different things. Or is it that
CC is still using bridge-ips (I don't know the current status of that;
see https://bugs.torproject.org/18167).

The reason I ask is I would like to explore differences between IPv4 and
IPv6 usage, and I'm wondering if I can just plug bridge-ip-versions into
the place of bridge-ip-transports in whatever calculation is necessary.
(E.g. for a given relay or country, I want a graph showing lower and
upper bounds for IPv4 and IPv6, just like we have now for transports.)