script: add bridgedb-metrics to BridgeDB collector metrics
To be able to deprecate BridgeDB collector needs to have the same metrics we currently have in BridgeDB. This is a service that collects prometheus metrics from all the distributors and produces a collector file with the BridgeDB format: https://gitlab.torproject.org/tpo/anti-censorship/bridgedb/-/blob/main/doc/bridgedb-metrics-spec.txt
- Closes: #188 (closed)