@Test public void toBuilder() {

  HttpTracing rpcTracing = HttpTracing.newBuilder(tracing).build();
  assertThat(rpcTracing.toBuilder().build())
    .usingRecursiveComparison()
    .isEqualTo(rpcTracing);
  assertThat(rpcTracing.toBuilder().clientSampler(neverSample()).build())
    .usingRecursiveComparison()
    .isEqualTo(HttpTracing.newBuilder(tracing).clientSampler(neverSample()).build());
}