code: func (file *GridFile) SetMeta(metadata interface{}) {
  	file.assertMode(gfsWriting)
  	data, err := bson.Marshal(metadata)
  	file.m.Lock()
  	if err != nil && file.err == nil {
  		file.err = err
  	} else {
  		file.doc.Metadata = &bson.Raw{Data: data}
  	}
  	file.m.Unlock()
  }
docs: // SetMeta changes the optional "metadata" field associated with the
  // file. The meaning of keys under that field is user-defined.
  // For example:
  //
  //     file.SetMeta(bson.M{"inode": inode})
  //
  // It is a runtime error to call this function when the file is not open
  // for writing.