refactor(exporter-prometheus): remove unnecessary isNaN() check

[cjihrig]

Which problem is this PR solving?

Not a problem, but the valueString() function can be simplified a bit.

Fixes N/A

Short description of the changes

This commit removes special handling of NaN values when converting JavaScript numbers to Prometheus strings. The NaN case can be combined with the case for finite numbers, as JavaScript already stringifies NaN to the expected value.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Ran the test suite and added an additional test case.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.63%. Comparing base (bdc0e3a) to head (645c2f4).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5377      +/-   ##
==========================================
+ Coverage   94.62%   94.63%   +0.01%     
==========================================
  Files         318      318              
  Lines        8038     8035       -3     
  Branches     1687     1685       -2     
==========================================
- Hits         7606     7604       -2     
+ Misses        432      431       -1     

Files with missing lines	Coverage Δ
...ry-exporter-prometheus/src/PrometheusSerializer.ts	95.77% <100.00%> (+0.60%)	⬆️

[pichlermarc]

thanks 👍