[prometheus] Add support for external_labels

[adamchabin]

What this PR does / why we need it

Support for external_labels option under global section.

Which issue this PR fixes

When you use multiple Prometheus as agents you would like to add tags like tenant:

external_labels:
tenant: 'tenant1'

(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]

As shown above, you are setting the field nested in server, not in global. The schema has to include server.externalLabels instead. Please, remember to insert the new field in values.yaml as well with a short comment/reference as seen e.g. at server.remoteRead.

[zeritti]

The helper template is unnecessary.

Suggested change

	{{/*
	Define prometheus.server.global.externalLabels producing a list of externalLabels
	*/}}
	{{- define "prometheus.server.global.externalLabels" -}}
	{{ toYaml .Values.server.global.externalLabels }}
	{{- end -}}

[zeritti]

The managed field externalLabels cannot be present in server.global as the latter gets inserted in Prometheus' config file as is and can therefore only include valid configuration parameters (the change won't pass CI). This is why external_labels and any other Prometheus' global configuration parameter can currently be set directly through this field.

Suggested change

	{{- if $root.Values.server.global.externalLabels }}
	external_labels:
	{{- include "prometheus.server.global.externalLabels" $root | nindent 8 }}
	{{- end }}
	{{- with $root.Values.server.externalLabels }}
	external_labels: {{ toYaml . | nindent 8 }}
	{{- end }}

[zeritti]

As in the comment above, needs to be moved out of server.global in server.

Suggested change

	## https://prometheus.io/docs/prometheus/latest/configuration/configuration/
	##
	externalLabels: {}
	## https://prometheus.io/docs/prometheus/latest/configuration/configuration/
	##
	externalLabels: {}

[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.