Prometheus Metrics

[abkfenris]

Expose Prometheus metrics for Buoy Barn servers.

In addition to the default metrics provided by https://pypi.org/project/django-prometheus/ we can also look at collecting metrics on how out-of-date various time series are. We'll probably want to think about how they are grouped/tagged (both server and platform, do we care about the individual time series, or just the aggregate of how far behind they may be?).