Add probe_duration_seconds to metrics

Fixes issue #2.

probes/instrumentation.go

@@ -18,10 +18,18 @@ var (
 		},
 		[]string{"probe", "probeset"},
 	)
+
+	probeDuration = prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Name: "probe_duration_seconds",
+			Help: "Duration of the last probe execution.",
+		},
+		[]string{"probe", "probeset"},
+	)
 )
 
 func init() {
-	prometheus.MustRegister(probeSuccess, probeLastRun)
+	prometheus.MustRegister(probeSuccess, probeLastRun, probeDuration)
 }
 
 type promResultStore struct {
@@ -43,4 +51,5 @@ func (p *promResultStore) Push(result *Result) {
 	probeset := result.Spec.Labels["probeset"]
 	probeSuccess.WithLabelValues(result.Spec.Name, probeset).Set(success)
 	probeLastRun.WithLabelValues(result.Spec.Name, probeset).Set(float64(result.Start.Unix()))
+	probeDuration.WithLabelValues(result.Spec.Name, probeset).Set(result.Duration.Seconds())
 }