code: func (file *GridFile) GetMeta(result interface{}) (err error) {
  	file.m.Lock()
  	if file.doc.Metadata != nil {
  		err = bson.Unmarshal(file.doc.Metadata.Data, result)
  	}
  	file.m.Unlock()
  	return
  }
docs: // GetMeta unmarshals the optional "metadata" field associated with the
  // file into the result parameter. The meaning of keys under that field
  // is user-defined. For example:
  //
  //     result := struct{ INode int }{}
  //     err = file.GetMeta(&result)
  //     if err != nil {
  //         panic(err.String())
  //     }
  //     fmt.Printf("inode: %d\n", result.INode)
  //