Pull Prometheus metrics from rdsys's backend process

I just deployed the metrics of rdsys's backend and it's now time for our Prometheus instance to scrape them. As with bridgestrap, these metrics can be public.

I suggest we first add a ProxyPass directive to make them available:

ProxyPass /rdsys-backend-metrics http://127.0.0.1:7100/metrics
ProxyPass /rdsys-backend-metrics/ http://127.0.0.1:7100/metrics

Once that is done, please make our Prometheus instance scrape https://bridges.torproject.org/rdsys-backend-metrics