Support http/json in otlptracehttp

[sqyang94]

Resolves #8150

My initial impression to this issue was to just add a new option like WithEncoding to switch between protobuf and json. But after realizing that users can use OTEL_EXPORTER_OTLP_[TRACES_]* environment variables to control the configuration and the OTLP specification has already defined OTEL_EXPORTER_OTLP_[*_]PROTOCOL to support grpc, http/protobuf and http/json, I decided to add a new option type Protocol to control the protocol/encoding.

Let me know if this is the intended usage of the OTLP protocol.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: sqyang94 / name: Shiqi Yang (0a0a9ea, 3559472, 475c114, 513a35f, 5bd26a1, 6151fb1, 6f306b3, 977dd80, acf3b98, cb4aac9, ce1cc8c, e6ccf56, ed8180e, fb01e2a)

[codecov]

Codecov Report

❌ Patch coverage is 86.98795% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.0%. Comparing base (b4c3b6a) to head (475c114).

Files with missing lines	Patch %	Lines
.../internal/otlpjson/export_trace_service_request.go	85.9%	22 Missing and 22 partials ⚠️
exporters/otlp/otlptrace/otlptracehttp/client.go	69.6%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #8273    +/-   ##
======================================
  Coverage   82.9%   83.0%            
======================================
  Files        314     316     +2     
  Lines      24985   25385   +400     
======================================
+ Hits       20729   21081   +352     
- Misses      3882    3907    +25     
- Partials     374     397    +23     

Files with missing lines	Coverage Δ
...ace/otlptracegrpc/internal/otlpconfig/envconfig.go	90.3% <100.0%> (+1.9%)	⬆️
...trace/otlptracegrpc/internal/otlpconfig/options.go	94.6% <100.0%> (+0.5%)	⬆️
...ace/otlptracehttp/internal/otlpconfig/envconfig.go	90.3% <100.0%> (+1.9%)	⬆️
...trace/otlptracehttp/internal/otlpconfig/options.go	93.4% <100.0%> (+0.6%)	⬆️
...internal/otlpjson/export_trace_service_response.go	100.0% <100.0%> (ø)
exporters/otlp/otlptrace/otlptracehttp/options.go	100.0% <100.0%> (ø)
exporters/otlp/otlptrace/otlptracehttp/client.go	79.6% <69.6%> (-1.1%)	⬇️
.../internal/otlpjson/export_trace_service_request.go	85.9% <85.9%> (ø)

... and 3 files with indirect coverage changes

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[MrAlias]

This adds the expected http/json configuration path, but I don’t think the wire behavior is correct yet.

The main issue is that the new JSON path is using generic protobuf JSON marshaling/unmarshaling rather than OTLP/JSON’s mapping rules. That can make a spec-compliant collector reject trace exports, and it also makes the response path less forward-compatible than OTLP requires. The current tests mostly validate the new path against the repo’s own mock collector, which uses the same protojson behavior and can hide these interoperability problems.

[sqyang94]

Now I'm leaning towards WithEncoding in otlptracehttp as opposed to WithProtocol. It feels a bit weird to call it WithProtocol but only provide two options, as grpc definitely doesn't make sense to be here.

[sqyang94]

Renamed in cb4aac9

[MrAlias]

The OTLP/JSON correctness issues I raised earlier look addressed in the current revision. I just have one remaining doc nit inline.