Function build_export_json 

pub fn build_export_json(r: &SpanRecord, service_name: &str) -> Value

Build the OTLP-JSON ExportTraceServiceRequest for one span. The attribute keys are the OpenInference / gen_ai.* keys a downstream OTel store normalizes, so the span round-trips. Ints are encoded as strings (the protobuf int64 → JSON mapping OTLP-JSON uses).