Add additional information for metrics.md

[jamesmacwhite]

It might be me, but I thought that the metrics.md documentation could do with directly mentioning the specific endpoint that the Prometheus metrics are provided at and also providing an example scrape_config for Prometheus usage, if not using the demo example.

I had to end up going through the GitHub repository to find this information, so I thought it might be a good idea to include in the docs directly.

[simskij]

Thank you for your contribution! 🎉
@all-contributors add @jamesmacwhite for docs

[allcontributors]

@simskij

I've put up a pull request to add @jamesmacwhite! 🎉