[stable/jenkins] Make PrometheusRule Namespace configurable

[fl-max]

Is your feature request related to a problem? Please describe.
By default, and without customizations, Prometheus only looks for PrometheusRules and ServiceMonitors in its own Namespace. Currently, the ServiceMonitor Namespace is configurable but the PrometheusRule is not.

Describe the solution you'd like
Add support to allow PrometheusRule to be configured via .Values.master.prometheus.prometheusRuleNamespace

Example:

{{- if .Values.master.prometheus.prometheusRuleNamespace }}
  namespace: {{ .Values.master.prometheus.prometheusRuleNamespace }}
{{- else }}
  namespace: {{ template "jenkins.namespace" . }}
{{- end }}

Also, add these to the values.yaml file and Document them.

Describe alternatives you've considered
Modifying Prometheus to scan other Namespaces

Additional context

[torstenwalter]

@fl-max I like you suggestion. Could you create a PR for that?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Any further update will cause the issue/pull request to no longer be considered stale. Thank you for your contributions.

[scottrigby]

📢 stable/jenkins has moved to https://github.com/jenkinsci/helm-charts. If still applicable, please reopen the issue there. Thanks!