Prometheus Resources as Kustomize Component

[haggishunk]

Summary

Adds support for prometheus resources to be composed into a pomerium deployment using the kustomize components feature.

Background

The current resources can be composed as kustomize resources

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- https://github.com/pomerium/ingress-controller.git/config/default?ref=v0.23.0
- https://github.com/pomerium/ingress-controller.git/config/prometheus?ref=v0.23.0

However flux kustomizations only support one path which requires users to deploy two such objects.

Also, some users prefer not to use remote kustomize bases.

Use Cases

Kustomize local or remote bases:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- https://github.com/pomerium/ingress-controller.git/config/default?ref=<newtag>
components:
- https://github.com/pomerium/ingress-controller.git/config/components/prometheus?ref=<newtag>

Flux kustomization:

apiVersion: source.toolkit.fluxcd.io/v1
kind: GitRepository
metadata:
  name: pomerium-ingress-controller
spec:
  ref:
    tag: <newtag>
  url: https://github.com/pomerium/ingress-controller.git
---
apiVersion: kustomize.toolkit.fluxcd.io/v1beta2
kind: Kustomization
metadata:
  name: pomerium-ingress-controller
spec:
  sourceRef:
    kind: GitRepository
    name: pomerium-ingress-controller
  path: config/default
  components:
  - ../components/prometheus

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[CLAassistant]

All committers have signed the CLA.

[haggishunk]

Hi @wasaga would you be kind enough to review this PR please?