Support Serving Metrics On an Alternate Port

[dpeckett]

Problem

Currently the main method prometheus metrics are exposed for scraping is that they are served unauthenticated on the main port https://github.com/jupyter-server/jupyter_server/pull/344

If the notebook is exposed to the internet this obviously leaks a lot of information.

Proposed Solution

Jupyter server should support serving metrics unauthenticated on an alternate port eg. 9090. This pattern is used in most web services that export prometheus metrics.