@Test public void nextSpan_extractedTraceId() {

  TraceIdContext traceIdContext = TraceIdContext.newBuilder().traceId(1L).build();
  TraceContextOrSamplingFlags extracted = TraceContextOrSamplingFlags.create(traceIdContext);
  assertThat(tracer.nextSpan(extracted).context().traceId())
    .isEqualTo(1L);
}