add configurable metrics listening cli option

[HarukaMa]

Motivation

Add an snarkOS cli option to configure the listening host and port of the metrics exporter.

Although I think listening to 127.0.0.1 by default should be a better default value, I kept 0.0.0.0 to not break existing users (the previous default was hardcoded 0.0.0.0:9000). Same for the cli option name, although it's different from rest and cdn options. Lemme know if breaking backwards compatibility is acceptable there.

Test Plan

Tested locally

Related PRs

(Link any related PRs here)

[HarukaMa]

oops, forgot to clippy and there is an unused import