fix(metrics): serve lightweight /healthz on metrics port

erpc/init.go

@@ -127,7 +127,7 @@ func Init(
 				return appCtx
 			},
 			Addr:              fmt.Sprintf(":%d", *cfg.Metrics.Port),
-			Handler:           promhttp.Handler(),
+			Handler:           newMetricsHandler(),
 			ReadHeaderTimeout: 10 * time.Second,
 		}
 		go func() {
@@ -158,3 +158,25 @@ func Init(
 
 	return nil
 }
+
+// newMetricsHandler serves Prometheus on /metrics (and / for back-compat) and a
+// cheap /healthz|/health that does not Gather the registry.
+//
+// Historically promhttp.Handler() was mounted as the root handler, so every
+// path — including kubelet probes hitting /healthz — returned the full
+// exposition. On hot pods that payload is tens of MB and Gather contends with
+// scrapes; probe timeoutSeconds: 5 then fails and kubelet restarts the pod.
+func newMetricsHandler() http.Handler {
+	mux := http.NewServeMux()
+	metrics := promhttp.Handler()
+	ok := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("OK"))
+	})
+	mux.Handle("/healthz", ok)
+	mux.Handle("/health", ok)
+	mux.Handle("/metrics", metrics)
+	mux.Handle("/", metrics)
+	return mux
+}

erpc/metrics_handler_test.go

@@ -0,0 +1,57 @@
+package erpc
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestNewMetricsHandler_HealthzIsLightweight(t *testing.T) {
+	h := newMetricsHandler()
+
+	for _, path := range []string{"/healthz", "/health"} {
+		rr := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, path, nil)
+		h.ServeHTTP(rr, req)
+
+		if rr.Code != http.StatusOK {
+			t.Fatalf("%s: status=%d want 200", path, rr.Code)
+		}
+		body, _ := io.ReadAll(rr.Body)
+		if string(body) != "OK" {
+			t.Fatalf("%s: body=%q want OK", path, body)
+		}
+		ct := rr.Header().Get("Content-Type")
+		if strings.Contains(ct, "openmetrics") || strings.Contains(ct, "text/plain; version=") {
+			t.Fatalf("%s: got prometheus content-type %q; health must not Gather", path, ct)
+		}
+	}
+}
+
+func TestNewMetricsHandler_MetricsStillExposition(t *testing.T) {
+	h := newMetricsHandler()
+
+	for _, path := range []string{"/metrics", "/"} {
+		rr := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, path, nil)
+		h.ServeHTTP(rr, req)
+
+		if rr.Code != http.StatusOK {
+			t.Fatalf("%s: status=%d want 200", path, rr.Code)
+		}
+		body, _ := io.ReadAll(rr.Body)
+		if !strings.Contains(string(body), "# HELP") && !strings.Contains(string(body), "# TYPE") {
+			// Empty registry still usually emits process/go collectors via default registerer.
+			t.Fatalf("%s: expected prometheus exposition, got %q", path, truncate(string(body), 200))
+		}
+	}
+}
+
+func truncate(s string, n int) string {
+	if len(s) <= n {
+		return s
+	}
+	return s[:n] + "..."
+}