fix: legacy label endpoint route

[trevorwhitney]

What this PR does / why we need it:

#14350 introduced detected field values, and refactored our routing logic. In doing so I left out the /api bit in /api/prom/label/{name}/values as referenced here. This fixes that mistake.

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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