docs: private static String resolveFieldNumbers(
        Optional<Descriptor> optDescriptor, String fmt, Iterable<Integer> fieldNumbers) {
      if (optDescriptor.isPresent()) {
        Descriptor descriptor = optDescriptor.get();
        List<String> strings = Lists.newArrayList();
        for (int fieldNumber : fieldNumbers) {
          FieldDescriptor field = descriptor.findFieldByNumber(fieldNumber);
          strings.add(field != null ? field.toString() : String.format("%d (?)", fieldNumber));
        }
        return String.format(fmt, join(strings));
      } else {
        return String.format(fmt, join(fieldNumbers));
      }
    }