@Test public void toSpan_idempotent() {

  TraceContext incoming = TraceContext.newBuilder().traceId(1L).spanId(2L).sampled(true).build();
  TraceContext toSpan = tracer.toSpan(incoming).context();
  assertThat(toSpan).isNotSameAs(incoming);
  assertThat(tracer.toSpan(incoming).context()).isSameAs(toSpan);
}