public Builder toBuilder() {
    return new Builder(this);
  }
@Test void toBuilder() {
    HttpTracing httpTracing = HttpTracing.newBuilder(tracing).build();

    assertThat(httpTracing.toBuilder().build())
      .usingRecursiveComparison()
      .isEqualTo(httpTracing);

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