api: Serialize discovered labels into JSON directly in dropped targets (#13484)

Converted maps into labels.Labels to avoid a lot of copying of data which leads to very high memory consumption while opening the /service-discovery endpoint in the Prometheus UI

Signed-off-by: Leegin <114397475+Leegin-darknight@users.noreply.github.com>

web/api/v1/api.go

@@ -903,7 +903,7 @@ type ScrapePoolsDiscovery struct {
 // DroppedTarget has the information for one target that was dropped during relabelling.
 type DroppedTarget struct {
 	// Labels before any processing.
-	DiscoveredLabels map[string]string `json:"discoveredLabels"`
+	DiscoveredLabels labels.Labels `json:"discoveredLabels"`
 }
 
 // TargetDiscovery has all the active targets.
@@ -1063,7 +1063,7 @@ func (api *API) targets(r *http.Request) apiFuncResult {
 			}
 			for _, target := range targetsDropped[key] {
 				res.DroppedTargets = append(res.DroppedTargets, &DroppedTarget{
-					DiscoveredLabels: target.DiscoveredLabels().Map(),
+					DiscoveredLabels: target.DiscoveredLabels(),
 				})
 			}
 		}

web/api/v1/api_test.go

@@ -1439,14 +1439,14 @@ func testEndpoints(t *testing.T, api *API, tr *testTargetRetriever, es storage.E
 				},
 				DroppedTargets: []*DroppedTarget{
 					{
-						DiscoveredLabels: map[string]string{
-							"__address__":         "http://dropped.example.com:9115",
-							"__metrics_path__":    "/probe",
-							"__scheme__":          "http",
-							"job":                 "blackbox",
-							"__scrape_interval__": "30s",
-							"__scrape_timeout__":  "15s",
-						},
+						DiscoveredLabels: labels.FromStrings(
+							"__address__", "http://dropped.example.com:9115",
+							"__metrics_path__", "/probe",
+							"__scheme__", "http",
+							"job", "blackbox",
+							"__scrape_interval__", "30s",
+							"__scrape_timeout__", "15s",
+						),
 					},
 				},
 				DroppedTargetCounts: map[string]int{"blackbox": 1},
@@ -1488,14 +1488,14 @@ func testEndpoints(t *testing.T, api *API, tr *testTargetRetriever, es storage.E
 				},
 				DroppedTargets: []*DroppedTarget{
 					{
-						DiscoveredLabels: map[string]string{
-							"__address__":         "http://dropped.example.com:9115",
-							"__metrics_path__":    "/probe",
-							"__scheme__":          "http",
-							"job":                 "blackbox",
-							"__scrape_interval__": "30s",
-							"__scrape_timeout__":  "15s",
-						},
+						DiscoveredLabels: labels.FromStrings(
+							"__address__", "http://dropped.example.com:9115",
+							"__metrics_path__", "/probe",
+							"__scheme__", "http",
+							"job", "blackbox",
+							"__scrape_interval__", "30s",
+							"__scrape_timeout__", "15s",
+						),
 					},
 				},
 				DroppedTargetCounts: map[string]int{"blackbox": 1},
@@ -1547,14 +1547,14 @@ func testEndpoints(t *testing.T, api *API, tr *testTargetRetriever, es storage.E
 				ActiveTargets: []*Target{},
 				DroppedTargets: []*DroppedTarget{
 					{
-						DiscoveredLabels: map[string]string{
-							"__address__":         "http://dropped.example.com:9115",
-							"__metrics_path__":    "/probe",
-							"__scheme__":          "http",
-							"job":                 "blackbox",
-							"__scrape_interval__": "30s",
-							"__scrape_timeout__":  "15s",
-						},
+						DiscoveredLabels: labels.FromStrings(
+							"__address__", "http://dropped.example.com:9115",
+							"__metrics_path__", "/probe",
+							"__scheme__", "http",
+							"job", "blackbox",
+							"__scrape_interval__", "30s",
+							"__scrape_timeout__", "15s",
+						),
 					},
 				},
 				DroppedTargetCounts: map[string]int{"blackbox": 1},