helm: add headless service

[tpaschalis]

PR Description

This PR adds a headless service to our helm chart. Its main use case is for use with clustering, so that new pods can use it for an SRV lookup to fins the rest of the cluster peers to connect to.

It is a copy of the existing service definition but with a -headless suffix to the name and ClusterIP set explicitly to None.

Comparing the previous manifests (exported with helm template .) and the new ones gives:

$ diff manifests-old.yaml manifests-new.yaml
156a157,178
> # Source: grafana-agent/templates/service-headless.yaml
> apiVersion: v1
> kind: Service
> metadata:
>   name: RELEASE-NAME-grafana-agent-headless
>   labels:
>     helm.sh/chart: grafana-agent-0.13.0
>     app.kubernetes.io/name: grafana-agent
>     app.kubernetes.io/instance: RELEASE-NAME
>     app.kubernetes.io/version: "v0.33.1"
>     app.kubernetes.io/managed-by: Helm
> spec:
>   clusterIP: 'None'
>   selector:
>     app.kubernetes.io/name: grafana-agent
>     app.kubernetes.io/instance: RELEASE-NAME
>   ports:
>     - name: http-headless-service
>       port: 80
>       targetPort: 80
>       protocol: "TCP"
> ---

Which issue(s) this PR fixes

Related to #2861 and ease-of-use.

Notes to the Reviewer

Nothing to note.

PR Checklist

• CHANGELOG updated
• Documentation added
• Tests updated

[rfratto]

To keep the amount of deployed resources small, can you make this optional by adding a new setting to the values.yaml?

I also think it should be opt-in for now rather than opt-out, since you only really need this when you're clustering and when you're not using go-discover to do the clustering.

[rfratto]

Thinking about this more: what if we made the service toggleable and then just relied on turning the primary service into a headless service instead of adding a new service?