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()
}
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()
}
// 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.