loki-stack: Add release name to prometheus service name.

[rally25rs]

What this PR does / why we need it:

Previously the prometheus service name was hard-coded to prometheus-server which would cause a conflict if attempting to helm install more than one release or namespace. For example

helm install release-one
helm install release-two

Would result in

Error: release release-two failed: clusterroles.rbac.authorization.k8s.io "prometheus-server" already exists

This PR incorporates the {{ .Release.Name }} into the service name to make it consistent with other generated names.

Note that the prometheus.fullname template defined in _helpers.tpl is named specifically to override the one defined by the normal prometheus chart. This allows the prometheus chart and templates in loki-stack to use the same logic for getting the prometheus service name.

Which issue(s) this PR fixes:
Fixes #1169

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[cyriltovena]

LGTM

Thank you !