@Test public void toBuilder() {

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