Show prometheus internal metrics (like scrape time) in the dashboard

[demeringo]

Problem

It would be interresting during development phase to monitor prometheus scrape time (to assess performance of cloud-scanner itself)

Solution

See if we can internal prometheus metrics and if yes expose them in the demo dashboard:

• prometheus data size
• clouds scanner scrape duration
• others relevant metrics ?

https://prometheus.io/docs/prometheus/latest/getting_started/#configuring-prometheus-to-monitor-itself

Alternatives

Additional context or elements

Maybe cloud scanner could also return its internal metrics (inventory duration, impact gathering duration, a.s.o) if called with a specific parameter (that should probably be off by default).