add metric for latest stored round

chain/beacon/store.go

@@ -76,6 +76,7 @@ func (d *discrepancyStore) Put(b *chain.Beacon) error {
 	expected := chain.TimeOfRound(d.group.Period, d.group.GenesisTime, b.Round) * 1e9
 	discrepancy := float64(actual-expected) / float64(time.Millisecond)
 	metrics.BeaconDiscrepancyLatency.Set(float64(actual-expected) / float64(time.Millisecond))
+	metrics.LastBeaconRound.Set(float64(b.GetRound()))
 	d.l.Info("NEW_BEACON_STORED", b.String(), "time_discrepancy_ms", discrepancy)
 	return nil
 }

metrics/metrics.go

@@ -44,6 +44,11 @@ var (
 		Name: "beacon_discrepancy_latency",
 		Help: "Discrepancy between beacon creation time and calculated round time",
 	})
+	// LastBeaconRound is the most recent round (as also seen at /health) stored.
+	LastBeaconRound = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "last_beacon_round",
+		Help: "Last locally stored beacon",
+	})
 
 	// HTTPCallCounter (HTTP) how many http requests
 	HTTPCallCounter = prometheus.NewCounterVec(prometheus.CounterOpts{
@@ -159,6 +164,7 @@ func bindMetrics() error {
 		GroupDialFailures,
 		GroupConnections,
 		BeaconDiscrepancyLatency,
+		LastBeaconRound,
 	}
 	for _, c := range group {
 		if err := GroupMetrics.Register(c); err != nil {