Add additional service labels

[m4r1u2]

PR Description

Add helm option to set extra service lables.

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• [X] CHANGELOG.md updated
• [X] Documentation added
• [ ] Tests updated
• [ ] Config converters updated

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[tpaschalis]

Hey there, thanks for the contribution. Could you explain the rationale behind service layers? Is there a difference than using the normal grafana-agent.labels field here?

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