code: protected boolean parseUnknownField(
        CodedInputStream input,
        UnknownFieldSet.Builder unknownFields,
        ExtensionRegistryLite extensionRegistry,
        int tag) throws IOException {
      return unknownFields.mergeFieldFrom(tag, input);
    }
docs: Called by subclasses to parse an unknown field.
  @return {@code true} unless the tag is an end-group tag.