When you area monitoring server from cloud providers the exporter
IP/hostname is not the server IP, so add some constant labels is useful
on dashboards and alerts.
And is very important to use with `--disable-default-metrics` to add
info like `datname` and figure out the database name that you are
monitoring
This did in fact turn out to be an oversight in the error handling. Now, any
error in the initial connection path will always trip pg_up to be 0.
Fixes#160
This commit implements a massive refactor of the repository, and
moves the build system over to use Mage (magefile.org) which should
allow seamless building across multiple platforms.
