Round up node_exporter metrics usage

[mflendrich]

Given the upcoming node_exporter migration to a separate container, there is a risk of information loss (both for the operator/autoscaling use, and for the monitoring dashboards).

Acceptance Criteria

• A document listing metrics of use in operator machinery/autoscaling
• A document listing metrics of use in monitoring dashboards
• For each of the metrics above, confirmation that we're not losing them by migrating node-exporter to a separate container. And - in case of loss - existence of an action plan.

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