Update simple example to use custom registry

Signed-off-by: Jéssica Lins <jessicaalins@gmail.com>

examples/simple/main.go

@@ -19,6 +19,7 @@ import (
 	"log"
 	"net/http"
 	
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
@@ -26,6 +27,11 @@ var addr = flag.String("listen-address", ":8080", "The address to listen on for
 
 func main() {
 	flag.Parse()
-	http.Handle("/metrics", promhttp.Handler())
+
+	// Create non-global registry.
+	reg := prometheus.NewRegistry()
+
+	// Expose /metrics HTTP endpoint using the created custom registry.
+	http.Handle("/metrics", promhttp.HandlerFor(reg, promhttp.HandlerOpts{Registry: reg}))
 	log.Fatal(http.ListenAndServe(*addr, nil))
 }