Allow to override the cluster headless service port name for Istio support

[josh-thisisbud]

This will be needed for an Istio mesh based installation due to this issue with headless services + Istio - https://istio.io/latest/docs/ops/common-problems/network-issues/#503-error-while-accessing-headless-services

PR Description

Which issue(s) this PR fixes

Fixes #800

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[CLAassistant]

All committers have signed the CLA.

[josh-thisisbud]

What needs to be done to get this over the line? As it's a small change in the Helm Chart.
Are you expecting me to backport this to Grafana agent before this can be merged?

[josh-thisisbud]

@rfratto as you added the label, would you answer the above question for me?

[rfratto]

What needs to be done to get this over the line? As it's a small change in the Helm Chart.

Thanks for the ping, and sorry this has been sitting around. I'll review this right after sending this comment.

Are you expecting me to backport this to Grafana agent before this can be merged?

No, sorry; that label is for the maintainers to track what they need to backport after a PR has been merged into main. I wouldn't want contributors to have to go through the effort of backporting.

[rfratto]

Thanks! Two small comments but this should be ready to go soon.

[rfratto]

LGTM, thanks for working on this!