code: func NewMongoStore(uri string, cnf *Config, opts ...*options.ClientOptions) (*MongoStore, error) {
  	if len(opts) == 0 {
  		opts = defaultMongoDBConf()
  	}
  	opts = append(opts, options.Client().ApplyURI(uri))
  	client, err := mongo.NewClient(opts...)
  	if err != nil {
  		return nil, err
  	}
  	mongoStore := &MongoStore{client: client, cnf: defaultConfig(cnf)}
  	if err := mongoStore.init(); err != nil {
  		return nil, err
  	}
  	return mongoStore, nil
  }
docs: // NewMongoStore creates a new MongoDB store. If no MongoDB client options are defined,
  // the client will be configured with a majority read/write concern, 5 sec. write concern timeout and journal write acknowledgement.