OpenTelemetry ConsoleSpanExporter on both client and server, with the
W3C traceparent riding the envelope's
extensions["x-vendor.opentelemetry.tracecontext"] slot (spec §11).
python examples/host_tracing/server.py # terminal 1
python examples/host_tracing/client.py # terminal 2
Client success: at least one client span and one server span share the
same trace_id. Spans print to stdout on both sides — this is the only
example with stdout noise from instrumentation.