Integrate Prometheus metrics in the Rucio extension (and the Hub)

[Soap2G]

As part of the integration process of the Rucio extension into CERN's SWAN, it is required to have monitoring metrics being generated by the extension.

• Enable Prometheus metrics in the extension
  • See feat: add Prometheus metrics generation for server #43 rucio/jupyterlab-extension#44
• Enable JupyterHub to scrape metrics from servers and expose them to the Prometheus instance