Send replication slots metrics to Prometheus

[bern4rdelli]

Send the status and lag metrics to Prometheus.
We will add two alerts, one when the status is equal to zero for more than five minutes, and another when the lag (in MB) is greater than X mb (TBD);

The local results are:

# HELP postgresql_replication_slot_lag Total lag of the replication slots
# TYPE postgresql_replication_slot_lag gauge
postgresql_replication_slot_lag{database_name="uuid",slot_name="my_dummy_wal"} 499
# HELP postgresql_replication_slot_status State of the replication slots
# TYPE postgresql_replication_slot_status gauge
postgresql_replication_slot_status{database_name="uuid",slot_name="my_dummy_wal"} 0

[diogonicoleti]

LGTM