disable prometheus if not configured (#663)

core/prometheus/agent.go

@@ -7,10 +7,19 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/tal-tech/go-zero/core/logx"
+	"github.com/tal-tech/go-zero/core/syncx"
 	"github.com/tal-tech/go-zero/core/threading"
 )
 
-var once sync.Once
+var (
+	once    sync.Once
+	enabled syncx.AtomicBool
+)
+
+// Enabled returns if prometheus is enabled.
+func Enabled() bool {
+	return enabled.True()
+}
 
 // StartAgent starts a prometheus agent.
 func StartAgent(c Config) {
@@ -19,6 +28,7 @@ func StartAgent(c Config) {
 			return
 		}
 
+		enabled.Set(true)
 		threading.GoSafe(func() {
 			http.Handle(c.Path, promhttp.Handler())
 			addr := fmt.Sprintf("%s:%d", c.Host, c.Port)