Ideas: allow `eessi check` to generate prometheus ready outputs

[Crivella]

This is a follow-up on the discussion in the EESSI happy hour

Beside allowing eessi check to generate machine-readable output (eg JSON) it could be useful to also allow generating files to be exported with the filesystem exporter of Prometheus Node exporter.
(EG: allow to pass a map/schema that translates the JSON keys into metric names)
The map in particular would be useful for sites that already implement this to switch to the EESSI-cli from custom scripts they might have.

I think streamlining the capability of checking the status of an EESSI installation with Prometheus + Grafana could be useful as i would assume this would be a tool that many sites can use to monitor and alert about the health of their installation