Protobuf-free OTLP/JSON metric exporter #111

Parent

Part of #102

What to build

An OTLP/JSON metric exporter that maps from the SDK metric data straight to tokens and never references protobuf, reusing the resource, scope and attribute mapping established for traces.

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

Metrics do not go through a recordable; they convert from the SDK metric data model directly. Sums, gauges, histograms and exponential histograms each need their JSON shape, and integer data points must be emitted as decimal strings.

Acceptance criteria

• A metric exporter produces OTLP/JSON without referencing protobuf
• Sum, gauge, histogram and exponential histogram point types are covered
• Integer data points and nanosecond timestamps are emitted as decimal strings
• An equivalence test asserts byte-identical JSON against the protobuf path
• Resource, scope and attribute mapping is shared with traces, not duplicated
• The exporter builds with the nlohmann backend excluded

Blocked by

• Protobuf-free OTLP/JSON trace exporter #110
• Protobuf-free OTLP/HTTP send path #117