(some) Prometheus metrics for PG are no longer coming in

At around 16:15 UTC we stopped getting some metrics for the database. This somewhat matches the deploy start (which started around 15:50 UTC https://gitlab.slack.com/archives/C101F3796/p1523462034000663).

Captura_de_pantalla_2018-04-11_a_la_s__15.42.35

/cc @gl-infra @yorickpeterse @_stark