monitoring: Create prometheus rules with helm chart

[travisn]

Description of your changes:
The prometheus rules had been previously created if the cephcluster CR setting monitoring.enabled was set to true. The rules were not customizable and therefore not flexible enough. Now the rules are installed by the helm chart. To customize the rules, a post-processor can be applied to the helm chart.

This is proposed to replace the approach in #9503.

The rules can then be customized with a helm post-processor using tools such as kustomize. For example,

1. Extract the helm chart to a yaml:

helm template -f values.yaml rook-release/rook-ceph-cluster > cluster-chart.yaml

2. Run kustomize to update the desired prometheus rules:

kustomize build . > updated-chart.yaml
kubectl create -f updated-chart.yaml

There are many possible configurations for kustomize, but here is one example for the yamls would need to exist in the current directory:

kustimization.yaml

patches:
- path: severity.yaml
  target:
    group: monitoring.coreos.com
    kind: PrometheusRule
    name: prometheus-ceph-rules
    version: v1
resources:
- cluster-chart.yaml

severity.yaml: In this example, update the labels and the "for" statement in the first rule

- op: add
  path: /spec/groups/0/rules/0/labels
  value:
    my-label: foo
    severity: none
- op: add
  path: /spec/groups/0/rules/0/for
  value: 15m

Which issue is resolved by this Pull Request:
Resolves #9082, #9005

Checklist:

• Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
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• Reviewed the developer guide on Submitting a Pull Request
• Pending release notes updated with breaking and/or notable changes for the next minor release.
• Documentation has been updated, if necessary.
• Unit tests have been added, if necessary.
• Integration tests have been added, if necessary.

[travisn]

This yaml is directly copied from the ceph source repo. This way, we (hopefully) don't have to maintain any customizations to what is picked up from the ceph repo.

[yuvalman]

lgtm!

[mergify]

This pull request has merge conflicts that must be resolved before it can be merged. @travisn please rebase it. https://rook.io/docs/rook/latest/development-flow.html#updating-your-fork