public Builder toBuilder() {
    return new Builder(this);
  }
@Test 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());
  }