Allow an explicit value to be set for CustomResourceStateMetrics

[withnale]

What would you like to be added:

With Custom Resource metrics, at present it only seems possible to generate values based on path attributes, but often it is desirable to define the metric to be a certain value e.g. 1

Why is this needed:

Sometime we want to create metrics of a form similar to kube_pod_labels

[bavarianbidi]

Please see #1755 where a forked version is mentioned where it's possible to define some "info" metrics - maybe this fit your needs?