Expose Prometheus metrics

[matiasgarciaisaia]

We should add a Prometheus-compatible endpoint to Verboice exposing stats about numbers of calls in each state, probably per channel. This would allow us to better monitor an instance's state and detect anomalies (peaks of calls queued, calls not getting done as much as they should, etc).

It would be really useful in the context of using Verboice in Surveda surveys, but it's a nice feature on its own anyway.

See instedd/surveda#1472 and instedd/surveda#1498 for reference.