feat: add base Prometheus instrumentation

[prlanzarin]

• Instrument the application to export basic operational metrics
  alongside prom-client's default metrics. Current set of exposed metrics
  as well as instructions on how to enable it can be checked in
  /docs/prometheus.md.
  • The docs thing is new - didn't know where to put it. Feel free to suggest otherwise.
  • The custom metric set are more of a baseline thing - this can be extended
    with more important stuff later on

Depends on #8