[otlp] refactor constants

[TimothyMothra]

Towards #5730

Changes

• move Common constants from each signal file into separate file
• cleanup constants files, add summary with link to original proto file
• update serializer classes

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

Attention: Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.18%. Comparing base (b9be07a) to head (3ccb4a1).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ntation/Serializer/ProtobufOtlpMetricSerializer.cs	75.00%	1 Missing ⚠️
...entation/Serializer/ProtobufOtlpTraceSerializer.cs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6010      +/-   ##
==========================================
+ Coverage   86.11%   86.18%   +0.06%     
==========================================
  Files         257      257              
  Lines       11690    11690              
==========================================
+ Hits        10067    10075       +8     
+ Misses       1623     1615       -8     

Flag	Coverage Δ
unittests-Project-Experimental	86.06% <94.28%> (-0.01%)	⬇️
unittests-Project-Stable	86.14% <94.28%> (+0.39%)	⬆️
unittests-Solution	?
unittests-UnstableCoreLibraries-Experimental	?

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

Files with missing lines	Coverage Δ
...ementation/Serializer/ProtobufOtlpLogSerializer.cs	99.18% <100.00%> (ø)
...Implementation/Serializer/ProtobufOtlpTagWriter.cs	100.00% <100.00%> (ø)
...ntation/Serializer/ProtobufOtlpMetricSerializer.cs	99.15% <75.00%> (ø)
...entation/Serializer/ProtobufOtlpTraceSerializer.cs	92.72% <80.00%> (ø)

... and 5 files with indirect coverage changes