Add NTP exporter

This exporter gets the time from a NTP server and exposes the offset
between the remote and local system time.

collector/ntp.go

@@ -0,0 +1,56 @@
+// +build !nontp
+
+package collector
+
+import (
+	"flag"
+	"fmt"
+	"time"
+
+	"github.com/beevik/ntp"
+	"github.com/golang/glog"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+var (
+	ntpServer = flag.String("ntpServer", "", "NTP server to use for ntp collector.")
+	ntpDrift  = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: Namespace,
+		Name:      "ntp_drift_seconds",
+		Help:      "Time between system time and ntp time.",
+	})
+)
+
+type ntpCollector struct {
+}
+
+func init() {
+	Factories["ntp"] = NewNtpCollector
+}
+
+// Takes a config struct and prometheus registry and returns a new Collector exposing
+// the offset between ntp and the current system time.
+func NewNtpCollector(config Config) (Collector, error) {
+	if *ntpServer == "" {
+		return nil, fmt.Errorf("No NTP server specifies, see --ntpServer")
+	}
+	c := ntpCollector{}
+
+	if _, err := prometheus.RegisterOrGet(ntpDrift); err != nil {
+		return nil, err
+	}
+	return &c, nil
+}
+
+func (c *ntpCollector) Update() (updates int, err error) {
+	t, err := ntp.Time(*ntpServer)
+	if err != nil {
+		return updates, fmt.Errorf("Couldn't get ntp drift: %s", err)
+	}
+	drift := t.Sub(time.Now())
+	updates++
+	glog.V(1).Infof("Set ntp_drift_seconds: %f", drift.Seconds())
+	ntpDrift.Set(drift.Seconds())
+
+	return updates, err
+}