Load custom queries with postgres exporter

[idoqo]

We currently visualize postgres metrics in grafana using the dashboard at pg-exporter-dashboard.json.

This dashboard contains some metrics that don't seem to be loaded by default and show up as N/A on the dashboard. Some of such metrics are:

• Total Database Size (pg_database_size())
• PostgreSQL uptime (pg_postmaster_start_time()).

We can make these metrics available by starting postgres_exporter with the extend.query-path flag and pass in their custom queries file.