[repo] Replace some fields with auto properties

[SimonCropp]

Fixes #
Design discussion issue #

Changes

Please provide a brief description of the changes here.

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)

[TimothyMothra]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.39%. Comparing base (c44984e) to head (bb2844e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6104      +/-   ##
==========================================
- Coverage   86.40%   86.39%   -0.01%     
==========================================
  Files         257      257              
  Lines       11648    11640       -8     
==========================================
- Hits        10064    10056       -8     
  Misses       1584     1584              

Files with missing lines	Coverage Δ
...etheus.HttpListener/Internal/PrometheusExporter.cs	100.00% <ø> (ø)
src/OpenTelemetry/Metrics/ExportModesAttribute.cs	100.00% <ø> (ø)
src/OpenTelemetry/ReadOnlyFilteredTagCollection.cs	100.00% <ø> (ø)