feat(bouncer): add multiple improvements such as : ignore unused pool…

[SoulKyu]

Add multiple new features such as :

• Config file for parameters, we use systemd and with this feature the password is not in clear in the systemctl status.
• Add a cache system with a sync every 2 min. Its permit to get a huge performance improvement by not querying pgbouncer at each scrape intervale of prometheus which is very huge when you have more than 10k pools in pgbouncer.
• Add a parameters that permit to avoid scraping pools that are no longer used and that should be recycled at pgbouncer restart.
• Add some improvement on the database connection.
• Some logging which can be very usefull.

If you have any question regarding this MR we can check it together.

In term of performance we are now able to serve our prometheus server in less than 0.03sec where it was timeout before.

We now avoid to collect pools that we does not care about.

The cache system permit to not flood too much our pgbouncer and has a smarter approch.

Hope this MR will be accepted

Thanks for the huge works on this exporter