disable prometheus if not configured (#663)

rest/handler/prometheushandler.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/tal-tech/go-zero/core/metric"
+	"github.com/tal-tech/go-zero/core/prometheus"
 	"github.com/tal-tech/go-zero/core/timex"
 	"github.com/tal-tech/go-zero/rest/internal/security"
 )
@@ -34,6 +35,10 @@ var (
 // PrometheusHandler returns a middleware that reports stats to prometheus.
 func PrometheusHandler(path string) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
+		if !prometheus.Enabled() {
+			return next
+		}
+
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			startTime := timex.Now()
 			cw := &security.WithCodeResponseWriter{Writer: w}

rest/handler/prometheushandler_test.go

@@ -6,9 +6,26 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/tal-tech/go-zero/core/prometheus"
 )
 
-func TestPromMetricHandler(t *testing.T) {
+func TestPromMetricHandler_Disabled(t *testing.T) {
+	promMetricHandler := PrometheusHandler("/user/login")
+	handler := promMetricHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "http://localhost", nil)
+	resp := httptest.NewRecorder()
+	handler.ServeHTTP(resp, req)
+	assert.Equal(t, http.StatusOK, resp.Code)
+}
+
+func TestPromMetricHandler_Enabled(t *testing.T) {
+	prometheus.StartAgent(prometheus.Config{
+		Host: "localhost",
+		Path: "/",
+	})
 	promMetricHandler := PrometheusHandler("/user/login")
 	handler := promMetricHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)