Update the running as non-superuser section. This solves the problem that certain metrics didn't produce a value.

We don't need GIT_API_KEY to clone the assets branch, so don't require it.

This functionality was added in c225a1e1
but not documented.

for memory parameters for bytes.

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

Fixes #188.

Postgres can emit 32MB and 64MB units which were previously unhandled. Handle them.

Improve the documentation for `--disable-default-metrics` and update the listing of current flags.