prometheus: Add mgmt_pgraph_start_time_seconds metric

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

prometheus/prometheus.go

@@ -36,7 +36,8 @@ const DefaultPrometheusListen = "127.0.0.1:9233"
 type Prometheus struct {
 	Listen string // the listen specification for the net/http server
 
-	checkApplyTotal *prometheus.CounterVec // total of CheckApplies that have been triggered
+	checkApplyTotal        *prometheus.CounterVec // total of CheckApplies that have been triggered
+	pgraphStartTimeSeconds prometheus.Gauge       // process start time in seconds since unix epoch
 
 }
 
@@ -59,6 +60,14 @@ func (obj *Prometheus) Init() error {
 	)
 	prometheus.MustRegister(obj.checkApplyTotal)
 
+	obj.pgraphStartTimeSeconds = prometheus.NewGauge(
+		prometheus.GaugeOpts{
+			Name: "mgmt_graph_start_time_seconds",
+			Help: "Start time of the current graph since unix epoch in seconds.",
+		},
+	)
+	prometheus.MustRegister(obj.pgraphStartTimeSeconds)
+
 	return nil
 }
 
@@ -88,3 +97,13 @@ func (obj *Prometheus) UpdateCheckApplyTotal(kind string, apply, eventful, error
 	metric.Inc()
 	return nil
 }
+
+// UpdatePgraphStartTime updates the mgmt_graph_start_time_seconds metric
+// to the current timestamp.
+func (obj *Prometheus) UpdatePgraphStartTime() error {
+	if obj == nil {
+		return nil // happens when mgmt is launched without --prometheus
+	}
+	obj.pgraphStartTimeSeconds.SetToCurrentTime()
+	return nil
+}