Add prometheus metrics support

[aditya-konarde]

https://github.com/saady/sentry-prometheus

We can expose prometheus metrics via the statsd exporter.

@mmclanerh this seems like a quick win. Let me know if there's something I can help with :)

[xyntrix]

@aditya-konarde nice find! we'll want to get this captured in a jira story in the backlog, if it doesn't already exist. though, as this is OSIO specific, i'm not sure what priority this will take in terms of other things we have going on right now. originally errortracking via sentry was meant to be a temporary solution until upstream provided a supported errortracking capability via TBD distributed tracing solution.. yet, here we are. :)

i think last time i looked at internal metrics, it was focused on internal beancounters on how the webapp/workers/queues were performing.. so it was aimed at our management of sentry vs per-project/topic details. went with a very high level blackbox view (ex: https://docs.sentry.io/server/monitoring/). we can probably do a spike at least, to see what we can get and and what level of effort it would take to implement, as a feature set for customers of sentry.