@Test public void nextSpan_extractedTraceId_extra() {

  TraceIdContext traceIdContext = TraceIdContext.newBuilder().traceId(1L).build();
  TraceContextOrSamplingFlags extracted = TraceContextOrSamplingFlags.newBuilder(traceIdContext)
    .addExtra(1L).build();
  assertThat(tracer.nextSpan(extracted).context().extra())
    .containsExactly(1L);
}