Fix target metadata API for empty match_targets (#6303)

According to the documentation, the target metadata API accepts it,
if no value for match_target has been provided. This was not the case
in the implementation.

This commit make the API behave as described in the docs.

Signed-off-by: Tobias Guggenmos <tguggenm@redhat.com>

web/api/v1/api.go

@@ -674,10 +674,18 @@ func (api *API) targetMetadata(r *http.Request) apiFuncResult {
 		}
 	}
 
-	matchers, err := promql.ParseMetricSelector(r.FormValue("match_target"))
+	matchTarget := r.FormValue("match_target")
+
+	var matchers []*labels.Matcher
+
+	var err error
+
+	if matchTarget != "" {
+		matchers, err = promql.ParseMetricSelector(matchTarget)
 		if err != nil {
 			return apiFuncResult{nil, &apiError{errorBadData, err}, nil, nil}
 		}
+	}
 
 	metric := r.FormValue("metric")
 
@@ -688,7 +696,7 @@ func (api *API) targetMetadata(r *http.Request) apiFuncResult {
 				break
 			}
 			// Filter targets that don't satisfy the label matchers.
-			if !matchLabels(t.Labels(), matchers) {
+			if matchTarget != "" && !matchLabels(t.Labels(), matchers) {
 				continue
 			}
 			// If no metric is specified, get the full list for the target.