Add IPv6 relay graphs to metrics site
We'd like to track relay support for IPv6 in two graphs:
- relays that have a reachable IPv6 ORPort address in the consensus (this is what clients will eventually use to check if a relay supports IPv6, see #21637 (moved) and #20916 (moved))
- relays that have any IPv6 ports in their exit policy summary in their microdescriptor (this is what most clients use to check if an exit supports IPv6)
This will help us answer questions like:
"How many relays support IPv6?"