feat: add Prometheus labels

[ColdFire87]

Hi,

This PR adds labeling support for ServiceMonitor endpoints. Those labels end up on the jobs/targets, and can be used to group jobs. E.g. can add a department & team label:

serviceMonitor:
  enabled: true
  endpoints:
    relabelings:
      - targetLabel: department
        replacement: "Foo"
      - targetLabel: team
        replacement: "Bar"

This can then be used in Grafana dashboards to group jobs by labels (or add variables to filter):

group by(job) (up{team="Foo"})

EDIT: This is the same PR as #7720, but I had to sync the changes into my fork, which forced the closure of the previous PR.

[ColdFire87]

/assign @desaintmartin

[desaintmartin]

@maciaszczykm could you trigger CI pipeline?

[codecov]

Codecov Report

Merging #7749 (f88e3e0) into master (dfd20b4) will increase coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7749      +/-   ##
==========================================
+ Coverage   42.01%   42.19%   +0.17%     
==========================================
  Files         163      218      +55     
  Lines       10827    12099    +1272     
  Branches        0      179     +179     
==========================================
+ Hits         4549     5105     +556     
- Misses       6031     6709     +678     
- Partials      247      285      +38     

[desaintmartin]

Thanks! It works this time!
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ColdFire87, desaintmartin

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[ColdFire87]

thanks everyone 👍