Add an alert for missing Node Group labels

[lunkan93]

Proposed feature

The reworked capacity management relies on the elastisys.io/node-group=<node-group> label on the nodes to function. We should add some optional lower priority alert that can alert the Platform Administrator if a node is missing this label.

Proposed alternatives

The alternative is silently not getting capacity management metrics/alerts for any node missing this label, which can be difficult to notice.

Additional context

No response

Definition of done

• [ ] We have an optional alert that triggers if the elastisys.io/node-group=<node-group> is missing from a node