Update basic example to use custom registry

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

prometheus/doc.go

@@ -35,39 +35,51 @@
 //    	"github.com/prometheus/client_golang/prometheus/promhttp"
 //    )
 //
-//    var (
-//    	cpuTemp = prometheus.NewGauge(prometheus.GaugeOpts{
+//    type metrics struct {
+//    	cpuTemp  prometheus.Gauge
+//      hdFailures *prometheus.CounterVec
+//    }
+//
+//    func NewMetrics(reg prometheus.Registerer) *metrics {
+//      m := &metrics{
+//        cpuTemp: prometheus.NewGauge(prometheus.GaugeOpts{
 //          Name: "cpu_temperature_celsius",
 //          Help: "Current temperature of the CPU.",
-//    	})
-//    	hdFailures = prometheus.NewCounterVec(
+//        }),
+//        hdFailures: prometheus.NewCounterVec(
 //          prometheus.CounterOpts{
 //            Name: "hd_errors_total",
 //            Help: "Number of hard-disk errors.",
 //          },
 //          []string{"device"},
-//    	)
-//    )
-//
-//    func init() {
-//    	// Metrics have to be registered to be exposed:
-//    	prometheus.MustRegister(cpuTemp)
-//    	prometheus.MustRegister(hdFailures)
+//        ),
+//      }
+//      reg.MustRegister(m.cpuTemp)
+//      reg.MustRegister(m.hdFailures)
+//      return m
 //    }
 //
 //    func main() {
-//    	cpuTemp.Set(65.3)
-//    	hdFailures.With(prometheus.Labels{"device":"/dev/sda"}).Inc()
+//      // Create a non-global registry.
+//      reg := prometheus.NewRegistry()
 //
-//    	// The Handler function provides a default handler to expose metrics
-//    	// via an HTTP server. "/metrics" is the usual endpoint for that.
-//    	http.Handle("/metrics", promhttp.Handler())
+//      // Create new metrics and register them using the custom registry.
+//      m := NewMetrics(reg)
+//      // Set values for the new created metrics.
+//    	m.cpuTemp.Set(65.3)
+//    	m.hdFailures.With(prometheus.Labels{"device":"/dev/sda"}).Inc()
+//
+//    	// Expose metrics and custom registry via an HTTP server
+//    	// using the HandleFor function. "/metrics" is the usual endpoint for that.
+//    	http.Handle("/metrics", promhttp.HandlerFor(reg, promhttp.HandlerOpts{Registry: reg}))
 //    	log.Fatal(http.ListenAndServe(":8080", nil))
 //    }
 //
 //
 // This is a complete program that exports two metrics, a Gauge and a Counter,
 // the latter with a label attached to turn it into a (one-dimensional) vector.
+// It register the metrics using a custom registry and exposes them via an HTTP server
+// on the /metrics endpoint.
 //
 // Metrics
 //