[tor-relays] metrics: add `IPv6 Exit Address` in addition to `Exit Address`

[s7r]

Hello,

Currently we have Exit Address for relays that use a different IPv4 
address for `Exit` connections vs their ORPort IPv4 address.

Similarly, one relay *could* listen on a certain IPv6 address ORPort but 
use a different IPv6 address for v6 exiting. Wouldn't it be useful to 
add this data too?

This would tell us how many relays use one static IPv6 address for 
ORPort connections and temporary IPv6 addresses for exiting (using IPv6 
privacy extensions).

By looking at https://metrics.torproject.org/relays-ipv6.html one could 
assume that almost 50% of the IPv6 enabled relays (IPv6 ORPort) are exits.
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