Add new stats for v2 and v3 onion service traffic
The current onion service traffic statistic includes v2 and v3 services.
But they are distinguishable at the rend point: v2 services use TAP to extend, and v3 use ntor.
This is how we tag and count circuits for v2 and v3 in experimental PrivCount:
https://github.com/privcount/tor/commit/ab0ee674d72a8b86cf394ecb392c0e829a265cf6
(Sorry it's not a very clean commit, look for circ->used_legacy_circuit_handshake.)
We could add a similar flag to tor, and use that flag to produce two new statistics, v2 and v3 data.
Since there will be a small number of v3 onion sites, we might want to implement this using PrivCount in tor, so their rend points do not stand out. If we don't use PrivCount in tor, we should fix the stats noise bugs in #23061 (moved) before we implement this.