fix(metrics): Remove scraping duration timer, as it's unreliable

The Prometheus server (central component) will also record the scrape time for
this particular scrape. So we just keep that one.

Author: peanball

internal/agent/metrics/metrics_test.go

@@ -1078,36 +1078,6 @@ func TestHealthEndpointRedisDown(t *testing.T) {
 	}
 }
 
-// --- Scrape Duration Test ---
-
-func TestScrapeDurationMetric(t *testing.T) {
-	// Suppress expected "failed to connect" logs from collectors whose DBs aren't mocked
-	log.SetOutput(io.Discard)
-	defer log.SetOutput(os.Stderr)
-
-	mc := newMockConnector("CONFIG_DB")
-	// DeviceCollector will read DEVICE_METADATA
-	mc.mocks["CONFIG_DB"].ExpectHGetAll("DEVICE_METADATA|localhost").SetVal(map[string]string{
-		"mac": "aa:bb:cc:dd:ee:ff",
-	})
-	// InterfaceCollector will list ports
-	mc.mocks["CONFIG_DB"].ExpectKeys("PORT|*").SetVal([]string{})
-
-	srv := NewMetricsServer(":0", mc, nil, "")
-
-	req := httptest.NewRequest("GET", "/metrics", nil)
-	w := httptest.NewRecorder()
-	srv.Handler.ServeHTTP(w, req)
-
-	if w.Code != http.StatusOK {
-		t.Fatalf("expected 200, got %d", w.Code)
-	}
-	body := w.Body.String()
-	if !strings.Contains(body, "sonic_scrape_duration_seconds") {
-		t.Error("response missing sonic_scrape_duration_seconds metric")
-	}
-}
-
 // --- Error Handling Test ---
 
 func TestDeviceCollectorRedisDown(t *testing.T) {

internal/agent/metrics/server.go

@@ -6,7 +6,6 @@ package metrics
 import (
 	"fmt"
 	"net/http"
-	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
@@ -44,22 +43,12 @@ func NewMetricsServer(addr string, connector RedisConnector, versionInfo Version
 		}
 	}
 
-	// Scrape duration gauge — updated after each /metrics response is written,
-	// so it reports the duration of the previous scrape (not the current one).
-	// This is consistent with how node_exporter and similar exporters work.
-	scrapeDuration := prometheus.NewGauge(prometheus.GaugeOpts{
-		Name: "sonic_scrape_duration_seconds",
-		Help: "Duration of the last metrics scrape in seconds",
-	})
-	registry.MustRegister(scrapeDuration)
+	// NOTE: Scrape duration is not emitted by this exporter. Prometheus records
+	// scrape_duration_seconds server-side as part of every scrape automatically.
 
 	mux := http.NewServeMux()
 	handler := promhttp.HandlerFor(registry, promhttp.HandlerOpts{})
-	mux.Handle("GET /metrics", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		start := time.Now()
-		handler.ServeHTTP(w, r)
-		scrapeDuration.Set(time.Since(start).Seconds())
-	}))
+	mux.Handle("GET /metrics", handler)
 	mux.HandleFunc("GET /healthz", func(w http.ResponseWriter, r *http.Request) {
 		client, err := connector.Connect("CONFIG_DB")
 		if err != nil {