fixing prometheus rules integration for operator and additionalLabels

[skuethe]

Hi there,

this PR addresses multiple problems I was facing during evaluation deployment:

• the PrometheusRule custom resource was not recognized and automatically loaded from the Prometheus Operator (kube-prometheus-stack), because the release label is missing
• I was unable to use additionalLabels on that custom resource, because it was only implemented for the ServiceMonitor custom resource
• Trying to use additionalLabels on the ServiceMonitor custom resource did not work - there seems to be some type mixtures and implementation issues in the helm template

The fixes include:

• renaming prometheus.enableAlerting to prometheus.prometheusRule to more accurately represent the custom resource we deploy (file templates/prometheusrule.yml with kind: PrometheusRule)
• keep validating against .Values.prometheus.enableAlerting for backwards compatibility
• add additional configuration options to prometheus.prometheusRule (more or less same as used in prometheus.serviceMonitor)
• fixing usage of prometheus.serviceMonitor.additionalLabels
• updating all documentation I could find about these settings
• also updating scripts and CI files

I am unsure about the versioning you use for Chart.yaml (and if maybe you generate it from some CI pipeline?), so I did not yet bump the chart version. Give me a hint if I should do this manually.

[joshulyne]

I am unsure about the versioning you use for Chart.yaml (and if maybe you generate it from some CI pipeline?), so I did not yet bump the chart version. Give me a hint if I should do this manually.

this is automated through one of our github actions!

thanks so much for contributing/fixing a lot of this! lgtm!