expose a promethues metrics exporter

Let's add prometheus exporter. We can start with the same metrics we expose to collector:
https://gitlab.torproject.org/tpo/anti-censorship/bridgedb/-/blob/main/bridgedb/metrics.py

Another interesting metric is bridge requests to moat and https by user agent, to reduce the identifiable data I would strip anything after the first /, so we only keep the browser name but not the version, OS or any of those details.