Export metrics for CollecTor

Produce daily metrics for collector and expose them in https://[Bridgestrap URL]/collector. It produces metrics for each bridge tested and for the number of requests resolved by cached results.

Bridgestrap stores the metrics on a file once per day. In case of bridgestrap restarts it will loose all the metrics that hasn't being stored as they are kept in memory.

The implementation is heavily inspired on the snowflake broker metrics: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/blob/master/broker/metrics.go

Closes: #15 (closed)