@Override
  public String toString() {
    boolean traceHi = traceIdHigh != 0;
    char[] result = new char[traceHi ? 32 : 16];
    int pos = 0;
    if (traceHi) {
      writeHexLong(result, pos, traceIdHigh);
      pos += 16;
    }
    writeHexLong(result, pos, traceId);
    return new String(result);
  }
@Test void testToString() {
    assertThat(base.toBuilder().traceIdHigh(222L).build().toString())
      .isEqualTo("00000000000000de000000000000014d");
  }