Fix Prometheus exporter writing bool label values

[JamesNK]

Fixes #4822
Design discussion issue #

Changes

Prometheus exporter writes bool label values as lowercase true and false strings.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Merging #4823 (1c59639) into main (5cc1f39) will increase coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 1c59639 differs from pull request most recent head dbe03cf. Consider uploading reports for the commit dbe03cf to get more accurate results

@@            Coverage Diff             @@
##             main    #4823      +/-   ##
==========================================
+ Coverage   83.90%   83.91%   +0.01%     
==========================================
  Files         293      293              
  Lines       11998    12001       +3     
==========================================
+ Hits        10067    10071       +4     
+ Misses       1931     1930       -1     

Files Changed	Coverage
...heus.HttpListener/Internal/PrometheusSerializer.cs	100.00%

[utpilla]

@JamesNK Could you add a CHANGELOG entry for this?

[JamesNK]

Done