Populate bridgedb-metrics entry on https://metrics.torproject.org/collector.html

We have https://metrics.torproject.org/collector.html#bridgedb-metrics but there is nothing explained on how we parse or deal with those metrics. The spec for that is https://gitlab.torproject.org/tpo/anti-censorship/bridgedb/-/blob/main/doc/bridgedb-metrics-spec.txt. We should look over our code and compare it to the spec and make sure both match and then document the steps on collector.html.