disable prometheus if not configured (#663)

zrpc/internal/serverinterceptors/prometheusinterceptor_test.go

@@ -5,10 +5,25 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/tal-tech/go-zero/core/prometheus"
 	"google.golang.org/grpc"
 )
 
-func TestUnaryPromMetricInterceptor(t *testing.T) {
+func TestUnaryPromMetricInterceptor_Disabled(t *testing.T) {
+	interceptor := UnaryPrometheusInterceptor()
+	_, err := interceptor(context.Background(), nil, &grpc.UnaryServerInfo{
+		FullMethod: "/",
+	}, func(ctx context.Context, req interface{}) (interface{}, error) {
+		return nil, nil
+	})
+	assert.Nil(t, err)
+}
+
+func TestUnaryPromMetricInterceptor_Enabled(t *testing.T) {
+	prometheus.StartAgent(prometheus.Config{
+		Host: "localhost",
+		Path: "/",
+	})
 	interceptor := UnaryPrometheusInterceptor()
 	_, err := interceptor(context.Background(), nil, &grpc.UnaryServerInfo{
 		FullMethod: "/",