add client_addr to metric pg_stat_activity_max_tx_duration

[betanummeric]

implements #985

[SuperQ]

Hmm, this could impact cardinality very badly as things scale up. I'm not sure this would be acceptable for most users.

[betanummeric]

@SuperQ I don't think most users have so much more client hosts than postgres-users to make the client_addr-label problematic. But sure, this introduces another factor to the amount of monitoring data.
Would it be ok for you to make this change opt-in using a CLI flag?

[SuperQ]

An opt-in flag would be fine. The only issue is that this collector setup hasn't been moved to our new collector package, so it's possibly slightly difficult to implement here.

[sysadmind]

I don't know that we have had a good example of customizing a collector that has been ported to the collector package either. I remember another request noting that the constructors for collectors don't have an obvious pattern for customization. @SuperQ Do we have a good example from another exporter for customizing collectors?