Move pg_stat_replication queries to collector package

[SuperQ]

Proposal

There are existing queries for pg_stat_replication in cmd/postgres_exporter/queries.go. These metrics should be migrated to the collector package.

[Efiartes]

This affects replication monitoring in the way that if only pg_up and pg_replication_lag_seconds are monitored in Secondary servers and there's a network outage between Primary and Secondary servers, Secondary servers get lagged without any alarm being triggered.

It seems more reasonable to monitor replication looking at Primary server data.
SELECT COUNT(*) FROM pg_stat_replication WHERE client_addr='SLAVE_IP' AND state = 'streaming';
If it returns 0, we have an unreachable Secondary server.

SELECT COALESCE(EXTRACT(EPOCH FROM replay_lag)::bigint, 0) AS replay_lag FROM pg_stat_replication WHERE client_addr='SLAVE_IP';
If it returns more than X we have a lagged Secondary server.