fix golint issues in core/stat (#515)

* change to use ServiceGroup to make it more clear

* fix golint issues in core/stat

core/stat/metrics_test.go

@@ -6,9 +6,14 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/tal-tech/go-zero/core/logx"
 )
 
 func TestMetrics(t *testing.T) {
+	logx.Disable()
+	DisableLog()
+	defer logEnabled.Set(true)
+
 	counts := []int{1, 5, 10, 100, 1000, 1000}
 	for _, count := range counts {
 		m := NewMetrics("foo")