feat: Decouple prost from otlp json

[jay-dee7]

Fixes #3419
Design discussion issue (if applicable) #

Changes

Once prost has generated the stubs, we process them to include conditional cfg-attr blocks so that OTLP/JSON-only implementation doesn't pull-in prost, properly respecting the defined cargo feature-set.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: jay-dee7 / name: jay-dee7 (032b343, 3131271, 51df745)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (67e7dff) to head (032b343).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3451   +/-   ##
=====================================
  Coverage   83.2%   83.2%           
=====================================
  Files        128     128           
  Lines      25048   25048           
=====================================
+ Hits       20859   20860    +1     
+ Misses      4189    4188    -1     

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