feat: move metrics server config to the policy file

[Xe]

This changes how the metrics server works so that it's defined in the policy file instead of in command line flags. As a transition mechanism, command line flags will be translated to the internal values the policy file parser creates.

Binding to a TCP socket looks like this:

metrics:
  bind: ":9090"
  network: "tcp"

If you want to bind metrics to a Unix socket, make sure to set the network to unix and add a socket mode:

metrics:
  bind: "/tmp/anubis_metrics.sock"
  network: unix
  socketMode: "0700" # must be a string

Checklist:

• Added a description of the changes to the [Unreleased] section of docs/docs/CHANGELOG.md
• Added test cases to the relevant parts of the codebase
• Ran integration tests npm run test:integration (unsupported on Windows, please use WSL)
• All of my commits have verified signatures