Introduce a endpoint with a prometheus compatible format for monitoring

[daniel-abramov]

This would allow us to monitor the state of the SFU:

• Amount of calls running on the SFU.
• Participants in a call.
• (not-for-production?) who is subscribed to whom.
• (not-for-production?) Parameters of each connection.

I.e. like webrtc-internals in the browser but for the SFU.