/**
  * @param \google\protobuf\FieldDescriptorProto $fieldDescriptor
  *
  * @return boolean
  */
 protected function isProtobufExtension(FieldDescriptorProto $fieldDescriptor)
 {
     if (!$fieldDescriptor->hasExtendee()) {
         return false;
     }
     $extendee = trim($fieldDescriptor->getExtendee(), '.');
     $package = 'google.protobuf';
     $length = strlen($package);
     return substr($extendee, 0, $length) === $package;
 }