Separate metrics server into a separate struct

[samlaf]

Is your feature request related to a problem? Please describe.

Right now the metrics interface and its implementation combine 2 separate concerns that should be separate:

• one struct to keep the prometheus variables and helper functions to change them
• actual metric server that listens on 9090

Problem comes when multiple services want to register their own metrics.
Each should have its own Metrics struct to keep track of its prom state and change them, but shouldn't also be a metrics server, because then its confusing where the separation of concern is.

Describe the solution you'd like

We should have a MetricsServer struct whose only purpose is to take in a registry (or many?) and that has a single StartServer() function to start its server.

Describe alternatives you've considered

Additional context