Implement list and fix test

mongo.go

@@ -55,7 +55,36 @@ func NewMongo(addr string, auth ...string) (*Mongo, error) {
 }
 
 func (mg *Mongo) List(ns []string, limits ...string) ([]string, error) {
-	return nil, errors.New("not impl")
+	namespace := resolveNamespace(ns)
+	limits = resolveLimits(limits)
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	collection := mg.db.Database(DefaultNamespace).Collection(namespace)
+	filter := bson.M{"_id": bson.M{
+		"$gte": limits[0],
+		"$lte": limits[1],
+	}}
+	projection := bson.M{"_id": 1}
+	cursor, err := collection.Find(ctx, filter, options.Find().SetProjection(projection))
+	if err != nil {
+		return nil, err
+	}
+	defer cursor.Close(ctx)
+	keys := []string{}
+	for cursor.Next(ctx) {
+		var elem bson.Raw
+		if err := cursor.Decode(&elem); err != nil {
+			return nil, err
+		}
+		if raw, err := elem.LookupErr("_id"); err != nil {
+			return nil, err
+		} else if s, ok := raw.StringValueOK(); !ok {
+			return nil, errors.New("_id is not a string")
+		} else {
+			keys = append(keys, s)
+		}
+	}
+	return keys, nil
 }
 
 func (mg *Mongo) Get(key string, ns ...string) ([]byte, error) {