Add programmatic way to extract metrics port when passing `0` in config

[Sushisource]

Is your feature request related to a problem? Please describe. When using the repo as a library, as in the cli, there's no good way (that I could find) to extract the bound metrics port if you pass in 0 in the config. This makes it hard to figure out (log, in my case) what port was actually chosen. I had to find a free port first to work around that, even though the code would do that for me.

Describe the solution you'd like Have some easy way to fetch the chosen metrics port after instantiating / starting a dev server