dual stack bridges only publish an IPv4 bridge line
Bridges currently only publish one bridge line per transport, and, if they have both an IPv4 and v6 address available, they always prefer IPv4, leaving the IPv6 address unused.
@arma checked the extra-info descriptor of one of my (dual stack) bridges and confirmed it only used its IPv4 address in the transport line. I don't see any IPv6 users either, but that's just anecdotal evidence. I think this has the potential to add a lot of new bridge address to the pool, which would be pretty neat from an anti-censorship perspective.
Right now I'll try to patch a bridge of mine to publish two transport lines in extra-info, one per IP version, and see if that breaks anything. If it doesn't, I can submit a MR after !782 gets merged (it affects the same area of the code).