code: private String getCustSerializedName(FieldOptions options, String defaultName) {
      for (Extension<FieldOptions, String> extension : serializedNameExtensions) {
        if (options.hasExtension(extension)) {
          return options.getExtension(extension);
        }
      }
      return protoFormat.to(jsonFormat, defaultName);
    }
docs: Retrieves the custom field name from the given options, and if not found, returns the specified
  default name.