OTLP/HTTP span exporter.
Ships the shape-only OTLP ResourceSpans payloads built by
[osproxy_observe::resource_spans] to a collector's /v1/traces endpoint
over HTTP with Content-Type: application/json (the OTLP/HTTP JSON binding).
Export is read-only and never on the request's critical path (docs/05,
ADR-005): [SpanExporter::export] returns immediately and the actual POST
runs in a spawned task whose result is ignored, a slow or down collector can
never add latency to, or fail, a client request. Telemetry is best-effort by
construction.