Add a collector for pending restarts

[mpldr]

Produces something along the lines of:

# HELP pg_catalog_pending_restart_info PostgreSQL configuration parameters that are pending a server restart to take effect.
# TYPE pg_catalog_pending_restart_info gauge
pg_catalog_pending_restart_info{pending_value="300",setting="max_connections"} 1

To test:

ALTER SYSTEM SET max_connections = '200'; SELECT pg_reload_conf();

[MarcWort]

Hi, I just saw this PR, and it is exactly what I need, thank you for creating this @mpldr.

Is there anything blocking this from being reviewed or merged? I’d be happy to help test or make updates if needed.