Stop converting unit "1" to "ratio" in Prometheus exporter

[anuq]

Summary

• Remove initUnit("1", "ratio") mapping from PrometheusUnitsHelper
• Treat unit "1" as unitless (return null from convertUnit), including after curly-brace annotation stripping (e.g. "1{dropped}")
• Update tests to reflect the new behavior

Aligns with spec change in open-telemetry/opentelemetry-specification#4966.

Fixes #8237

[linux-foundation-easycla]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (207c861) to head (05d640a).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8252      +/-   ##
============================================
- Coverage     90.32%   90.32%   -0.01%     
- Complexity     7651     7653       +2     
============================================
  Files           842      842              
  Lines         23075    23074       -1     
  Branches       2312     2312              
============================================
- Hits          20842    20841       -1     
  Misses         1514     1514              
  Partials        719      719              

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[jack-berg]

Please sign the CLA - thanks!

[breedx-splk]

LGTM thanks!