WIP: Resolve "postgres-exporter error after upgrade"

Test plan:

• With gitlab_rails['db_host'] at default value
  • postgres-exporter works on a fresh install
  • postgres-exporter works on upgrade
  • postgres-exporter works after setting gitlab_rails['db_host'] to a different(valid) value
• With gitlab_rails['db_host'] at non-default value
  • postgres-exporter works on a fresh install
  • postgres-exporter works on an upgrade
  • postgres-exporter works after setting gitlab_rails['db_host'] to a different(valid) value

Closes #3911 (closed)