fix the behavior of empty values

Signed-off-by: h-otter <[email protected]>

Author: h-otter

Diff for: injectproxy/routes.go

@@ -539,8 +539,13 @@ func (r *routes) matcher(w http.ResponseWriter, req *http.Request) {
 		Value: labelValuesToRegexpString(MustLabelValues(req.Context())),
 	}
 
-	var q url.Values
-	if req.Method == http.MethodPost && req.Header.Get("Content-Type") == "application/x-www-form-urlencoded" {
+	q := req.URL.Query()
+	if err := injectMatcher(q, matcher); err != nil {
+		return
+	}
+
+	req.URL.RawQuery = q.Encode()
+	if req.Method == http.MethodPost {
 		if err := req.ParseForm(); err != nil {
 			return
 		}
@@ -555,18 +560,16 @@ func (r *routes) matcher(w http.ResponseWriter, req *http.Request) {
 		newBody := q.Encode()
 		req.Body = io.NopCloser(strings.NewReader(newBody))
 		req.ContentLength = int64(len(newBody))
-	} else {
-		q = req.URL.Query()
-		if err := injectMatcher(q, matcher); err != nil {
-			return
-		}
 	}
 
-	req.URL.RawQuery = q.Encode()
 	r.handler.ServeHTTP(w, req)
 }
 
 func injectMatcher(q url.Values, matcher *labels.Matcher) error {
+	if len(q) == 0 {
+		return nil
+	}
+
 	matchers := q[matchersParam]
 	if len(matchers) == 0 {
 		q.Set(matchersParam, matchersToString(matcher))