add metrics for group size and threshold (#771)

fix #767

Co-authored-by: Nicolas Gailly <nikkolasg@users.noreply.github.com>

chain/beacon/store.go

@@ -77,6 +77,8 @@ func (d *discrepancyStore) Put(b *chain.Beacon) error {
 	discrepancy := float64(actual-expected) / float64(time.Millisecond)
 	metrics.BeaconDiscrepancyLatency.Set(float64(actual-expected) / float64(time.Millisecond))
 	metrics.LastBeaconRound.Set(float64(b.GetRound()))
+	metrics.GroupSize.Set(float64(d.group.Len()))
+	metrics.GroupThreshold.Set(float64(d.group.Threshold))
 	d.l.Info("NEW_BEACON_STORED", b.String(), "time_discrepancy_ms", discrepancy)
 	return nil
 }

metrics/metrics.go

@@ -38,6 +38,14 @@ var (
 		Name: "group_connections",
 		Help: "Number of peers with current GrpcClient connections",
 	})
+	GroupSize = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "group_size",
+		Help: "Number of peers in the current group",
+	})
+	GroupThreshold = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "group_threshold",
+		Help: "Number of shares needed for beacon reconstruction",
+	})
 	// BeaconDiscrepancyLatency (Group) millisecond duration between time beacon created and
 	// calculated time of round.
 	BeaconDiscrepancyLatency = prometheus.NewGauge(prometheus.GaugeOpts{
@@ -163,6 +171,8 @@ func bindMetrics() error {
 		APICallCounter,
 		GroupDialFailures,
 		GroupConnections,
+		GroupSize,
+		GroupThreshold,
 		BeaconDiscrepancyLatency,
 		LastBeaconRound,
 	}