chore: simplify prometheus check (#3672)

core/metric/metric.go

@@ -1,5 +1,7 @@
 package metric
 
+import "github.com/zeromicro/go-zero/core/prometheus"
+
 // A VectorOpts is a general configuration.
 type VectorOpts struct {
 	Namespace string
@@ -8,3 +10,11 @@ type VectorOpts struct {
 	Help      string
 	Labels    []string
 }
+
+func update(fn func()) {
+	if !prometheus.Enabled() {
+		return
+	}
+
+	fn()
+}