Add /metrics endpoint server to the proxy

[hawkw]

This PR adds a Hyper server that serves the proxy /metrics endpoint. It currently doesn't serve anything because no scrapable metrics are collected. The metrics server's address and port can be configured with the CONDUIT_PROXY_METRICS_LISTENER environment variable.

$ http get localhost:4191/metrics                   
HTTP/1.1 200 OK
Date: Thu, 08 Mar 2018 23:09:23 GMT
Transfer-Encoding: chunked


$ http get localhost:4191          
HTTP/1.1 404 Not Found
Content-Length: 0
Date: Thu, 08 Mar 2018 23:09:36 GMT

Closes #540

[olix0r]

s/scrapable/prometheus/ ?

[seanmonstar]

I'd consider calling this file prometheus, since it is specifically going to be in prometheus format, and scrape might be less understandable immediately for Future Us.