Federate non-namespaced metrics

[adenitiu]

How to categorize this PR?

/area monitoring
/kind enhancement

What this PR does / why we need it:
This PR enables the federation of non-namespaced metrics to the aggregate-prometheus. Metrics that would be added are:

kube_node_info
kube_node_labels
kube_node_spec_taint
kube_node_spec_unschedulable
kube_node_status_allocatable
kube_node_status_capacity
kube_node_status_condition
scrape_duration_seconds
scrape_samples_post_metric_relabeling
scrape_samples_scraped
scrape_series_added
up

Which issue(s) this PR fixes:
The interest of this PR is to have more node specific metrics available at the aggregate level.

Special notes for your reviewer:
Discussed this approach with @istvanballok and @rickardsjp .

Release note:

Federate non-namespaced metrics, e.g. kube_node_spec_taint, kube_node_spec_unschedulable. 

[istvanballok]

/lgtm

[gardener-prow]

LGTM label has been added.

Git tree hash: 80077c786700e0e09f869f07b1fc3ca393045ccc

[gardener-prow]

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