func (s *LogStorage) AppLogCollection(appName string) *storage.Collection {
	if appName == "" {
		return nil
	}
	return s.Collection("logs_" + appName)
}
func (s *LogStorage) AppLogCollection(appName string) *storage.Collection {
	if appName == "" {
		return nil
	}
	return s.Collection("logs_" + appName)
}
// AppLogCollection returns the logs collection for one app from MongoDB.