PTs should pass user count events back to Tor
Right now relays and bridges publish aggregate per-country stats in their extrainfo descriptors, like
dirreq-v3-ips us=5768,ru=1952,id=1584,de=912,in=696,[...](And in the future they will add noise to them in a smarter way, like via privcount, but that's orthogonal to this ticket.)
It would be great to have per-transport stats to go with this data.
One approach would be for the PT to send back an event when it hears about a user, or when a user does a countable thing like fetching a consensus. And then Tor can aggregate them according to however it thinks is best.
(We could have the PT do the aggregation, but (a) teaching the PT about geoip lookups when it otherwise doesn't need them doesn't sound so fun, and (b) teaching each PT how to add noise appropriately seems like it scales poorly compared to doing it once, well, inside Tor.)
Related question: what stats does https://metrics.torproject.org/userstats-bridge-transport.html use currently?
This ticket motivated by #29111 (moved).