Say which relative URL roots the metrics path decode cannot fix

What does this MR do?

Adds the line suggested in the review of !9929 to the decode_relative_url comment: decoding the relative URL root restores only the characters Go escapes again when Prometheus builds the scrape URL, and Go escapes some characters a root holds as written. So a%2Cb is requested as a,b and my(gitlab) as my%28gitlab%29, which Puma answers with a 404. The Linux package cannot correct that, because Prometheus accepts no raw path.

Comment only: no behavior change.

Related issues

Follow-up to !9929 (merged). Related to #9933 (closed).

Checklist

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