Sanitize label input data

What does this MR do and why?

Sanitizes the labels to avoid passing invalid strings to prometheus.

Closes #370604 (closed) #381424 (closed)

Screenshots or screen recordings

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How to set up and validate locally

1. Send request to gdk.test/?8\u003c/x
2. Check curl -ks https://gdk.test/-/metrics | ./promtool check metrics
3. The promtool should not report any issues in parsing the data

MR acceptance checklist

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