Add argument for configuring Prometheus port

[kozistr]

What does this PR do?

Allow configuring Prometheus port via argument (and environment variable)

Fixes #499

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@Narsil, @alvarobartt

[Narsil]

LGTM, thanks for this !

[cebtenzzre]

this PR seems to break the CLI: #601