[chore] Small nits, consistent JSON and Proto logic to call into internal

[bogdandrutu]

The main idea is that the [*]otlp/Request is for http/grpc OTLP communication, but the Marshaler is only marshaling non-protocol specific data.

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.52%. Comparing base (5a5da91) to head (c6f69d2).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pdata/plog/plogotlp/request.go	72.72%	2 Missing and 1 partial ⚠️
pdata/pmetric/pmetricotlp/request.go	72.72%	2 Missing and 1 partial ⚠️
pdata/pprofile/pprofileotlp/request.go	72.72%	2 Missing and 1 partial ⚠️
pdata/ptrace/ptraceotlp/request.go	72.72%	2 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (72.72%) 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   #13724      +/-   ##
==========================================
- Coverage   91.66%   91.52%   -0.14%     
==========================================
  Files         639      640       +1     
  Lines       42423    42495      +72     
==========================================
+ Hits        38887    38894       +7     
- Misses       2710     2782      +72     
+ Partials      826      819       -7     

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