[prometheus-pushgateway] Add 'release' label to prometheus-pushgateway service monitor

[nbelov-magdv]

'release' label is required for integrating with kube-prometheus-stack

What this PR does / why we need it

Adds 'release' label to prometheus-pushgateway ServiceMonitor.
This label is used by prometheus from kube-prometheus-stack for collecting metrics.

I cannot use {{ .Release.Name }} as value in values.yaml. So I've add it to template.

Also, I don't understand why kube-prometheus-stack uses 'release' label, but not 'app.kubernetes.io/instance' label.
But I am not good in k8s.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])

[zeritti]

Thank you, @nbelov-magdv, for your PR.

'release' label is required for integrating with kube-prometheus-stack

It is not entirely true. By default, Prometheus operator as deployed through kube-prometheus-stack sets a service monitor selector to release: RELEASE_NAME. Keeping it is not required, though, as this service monitor selector can be cleared by means of prometheus.prometheusSpec.serviceMonitorSelectorNilUsesHelmValues=false and, optionally, a custom selector can be set through prometheus.prometheusSpec.serviceMonitorSelector (serviceMonitorSelector).

If such a selector is set in prometheus CR, either default or custom, the corresponding label can be set in pushgateway chart's serviceMonitor.additionalLabels. Adding the release label to all manifests' labels and selectors by default seems unnecessary. We could, however, consider optionally inserting the release label in labels through a boolean field e.g. releaseLabel.

[nbelov-magdv]

Thank you for good idea.
I've refactored patch: add 'serviceMonitor.releaseLabel' boolean parameter for adding 'release' label.