Serve prometheus on localhost

[alexg-axis]

The promhttp library is hard coded to serve the metrics on all interfaces. As such the metrics are open to all by default, with no way of being controlled. The underlying microhttpd supports several options, though. The prom library is however unmaintained and putting effort there to add additional options would likely not be a worth investment. I therefore suggest to implement the HTTP part of the library (basically a call to start the HTTP daemon and serve the /metrics endpoint) to coturn instead, so that we're free to add additional options that better suite the coturn use case. I'm willing to put in the required work.