Expose statistics using Prometheus's conventions

We'd be interesting in adding a `/metrics` page to warcprox that [exposed the proxy statistics in a form that Prometheus can use](https://prometheus.io/docs/instrumenting/exposition_formats/). It would [look something like](https://prometheus.io/docs/instrumenting/exposition_formats/):

```
warcprox_uris_total{"bucket":"__all__",kind="all"} 37
warcprox_wire_bytes{"bucket":"__all__",kind="all"} 1502781
warcprox_uris_total{"bucket":"__all__",kind="new"} 15
warcprox_wire_bytes{"bucket":"__all__",kind="new"} 1179906
warcprox_uris_total{"bucket":"__all__",kind="revisit"} 22
warcprox_wire_bytes{"bucket":"__all__",kind="revisit"} 322875
```

We could use [the official library](https://github.com/prometheus/client_python) to format the metrics but that's probably overkill as it might bring in a lot of additional dependencies.

If we put this together, would you be happy for us to add this feature?