add client_addr to pg_stat_activity_max_tx_duration metric

[betanummeric]

proposal

Add the client_addr field from pg_stat_activity as a label to the metric pg_stat_activity_max_tx_duration.

use case

My company runs the postgres_exporter for production PostgreSQL instances. Often the application (postgres client) is running on multiple hosts, sharing a common username. When one application instance blocks others by keeping an idle transaction open for too long, we need the client_addr information to figure out which application host is causing that.
By adding the client_addr field to the metrics, we would enable the application admins to determine the malfunctioning host without needing to ask a DBA (someone with privileges to query pg_stat_activity directly).