fix: support string compatible types in to_string

Signed-off-by: Charles-Edouard Brétéché <charles.edouard@nirmata.com>

Author: eddycharly

pkg/api/api_test.go

@@ -163,6 +163,22 @@ func TestSearch(t *testing.T) {
 			}},
 		},
 		want: 1.0,
+	}, {
+		args: args{
+			expression: "length(@[?to_string(metric.__name__) == 'foo'])",
+			data: []struct {
+				Metric map[Label]Label
+			}{{
+				Metric: map[Label]Label{
+					"__name__": "foo",
+				},
+			}, {
+				Metric: map[Label]Label{
+					"__name__": "bar",
+				},
+			}},
+		},
+		want: 1.0,
 	}, {
 		args: args{
 			expression: "length(@[?metric.__name__ == 'foo'])",

pkg/functions/functions.go

@@ -34,6 +34,8 @@ const (
 	JpAny         JpType = "any"
 )
 
+var stringType = reflect.TypeFor[string]()
+
 type FunctionEntry struct {
 	Name      string
 	Arguments []ArgSpec
@@ -695,6 +697,9 @@ func jpfToString(arguments []interface{}) (interface{}, error) {
 	if v, ok := arguments[0].(string); ok {
 		return v, nil
 	}
+	if argType := reflect.TypeOf(arguments[0]); argType.Kind() == stringType.Kind() {
+		return reflect.ValueOf(arguments[0]).Convert(stringType).Interface(), nil
+	}
 	result, err := json.Marshal(arguments[0])
 	if err != nil {
 		return nil, err