[Prometheus.HttpListener] Avoid array lookup

[martincostello]

Changes

• Avoid array lookup and implicit bounds checks by converting to a switch statement.
• Apply analyzer suggestions to simplify/modernize code.
• Improve test coverage for metric mapping (mostly written by Copilot).

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.27%. Comparing base (496e4ac) to head (d04cc43).
⚠️ Report is 12 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6851      +/-   ##
==========================================
+ Coverage   86.90%   87.27%   +0.36%     
==========================================
  Files         263      263              
  Lines       12382    12385       +3     
==========================================
+ Hits        10761    10809      +48     
+ Misses       1621     1576      -45     

Flag	Coverage Δ
unittests-Project-Experimental	87.20% <100.00%> (+0.47%)	⬆️
unittests-Project-Stable	87.17% <100.00%> (+0.35%)	⬆️
unittests-UnstableCoreLibraries-Experimental	89.42% <100.00%> (+3.35%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ometheus.HttpListener/Internal/PrometheusMetric.cs	99.31% <100.00%> (+25.18%)	⬆️

... and 4 files with indirect coverage changes

[Copilot]

Pull request overview

This PR optimizes the GetPrometheusType method by replacing array lookup with a switch statement, applies modern C# patterns to simplify method bodies, and significantly expands test coverage for metric unit mapping.

Changes:

• Replaced array lookup in GetPrometheusType with a switch statement to avoid bounds checks and improve clarity
• Converted several methods to expression-bodied members for modern, concise code
• Added comprehensive test coverage for unit mapping, metric type mapping, and edge cases

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusMetric.cs	Replaced array-based metric type lookup with switch statement; converted methods to expression-bodied members
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusMetricTests.cs	Added 225 lines of comprehensive unit tests covering metric unit mapping, type conversion, and edge cases

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