JsonReader and protobuf-free partial-success response parsing #113

Parent

Part of #102

What to build

Partial-success responses parsed and logged without protobuf, so the OTLP HTTP client no longer depends on the protobuf JSON-to-message path on its read side. Rejected counts and error messages stay visible, which matters on a device with no other diagnostic channel.

Target: opentelemetry-cpp fork, branch otlp-json-no-protobuf.

The reader is deliberately document-shaped rather than token-shaped, roughly five accessors. The asymmetry with the writer is intentional and should be stated explicitly when this is proposed upstream: the writer handles unbounded telemetry batches where streaming matters, while the reader handles a two-field response where a pull parser would be pure cost.

Acceptance criteria

• A minimal JsonReader interface exists with an nlohmann-backed default
• The response path parses partial-success fields without protobuf
• Rejected counts and error messages are logged through the SDK internal log handler
• A malformed or truncated response body is handled without crashing and without exceptions
• An empty or absent body is treated as full success, as before
• The asymmetry with the writer is documented in the interface

Blocked by

• Protobuf-free OTLP/JSON trace exporter #110