[BUGFIX] fix variable interpolation in prometheus label value query

[zhuje]

Description

Current (Broken) Format:
 GET /api/v1/label/job/values?start=1781076832&end=1781080432&namespace=%24%7Bnamespace%3Aqueryparam%7D
 Where %24%7Bnamespace%3Aqueryparam%7D is URL-encoded ${namespace:queryparam} (the raw, uninterpolated variable)

Expected (Fixed) Format:
GET /api/v1/label/job/values?start=1781076832&end=1781080432&namespace=a&namespace=b&namespace=c&namespace=d
 Where the namespace parameter is repeated for each selected value in the multi-select variable, just like the working query_range requests.

Screenshots

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• [N/A] Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.