[chore] [pdata] Avoid allocations when copy oneOf fields

[dmitryax]

Similar to #13290, for oneOf fields

[codecov]

Codecov Report

Attention: Patch coverage is 79.66102% with 12 lines in your changes missing coverage. Please review.

Project coverage is 91.57%. Comparing base (6746dda) to head (7b4c926).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pdata/pmetric/generated_exemplar.go	50.00%	6 Missing ⚠️
pdata/pmetric/generated_numberdatapoint.go	50.00%	6 Missing ⚠️

❌ Your patch check has failed because the patch coverage (79.66%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13291      +/-   ##
==========================================
- Coverage   91.64%   91.57%   -0.07%     
==========================================
  Files         522      522              
  Lines       29111    29146      +35     
==========================================
+ Hits        26678    26691      +13     
- Misses       1919     1938      +19     
- Partials      514      517       +3     

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[dmitryax]

contrib tests are fixed in open-telemetry/opentelemetry-collector-contrib#40978

[dmathieu]

Should the new code path be tested?

[dmitryax]

It's actually already covered. We just don't test every option for the primitive oneOf's. In copyOrigExemplar, we cover otlpmetrics.Exemplar_AsInt case. In copyOrigNumberDataPoint, we cover otlpmetrics.NumberDataPoint_AsDouble