As part of the overall modernization of this application, this commit
replaces the prometheus crate with the generic metrics facade crate, and
the metrics_exporter_prometheus crate for exporting the metrics to
prometheus.
This also removes some useless http metrics that were cargo-culted in
from the prometheus crate example.
BREAKING CHANGE: http metrics are removed by this change
There are substructs for the different parts of the config now, as a
preparation of a larger refactor coming in.
BREAKING CHANGE: config file format changed