[#1054] neofs-node: add epoch metric

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/metrics/metrics.go

@@ -1,10 +1,13 @@
 package metrics
 
+import "github.com/prometheus/client_golang/prometheus"
+
 const namespace = "neofs_node"
 
 type StorageMetrics struct {
 	objectServiceMetrics
 	engineMetrics
+	epoch prometheus.Gauge
 }
 
 func NewStorageMetrics() *StorageMetrics {
@@ -14,8 +17,22 @@ func NewStorageMetrics() *StorageMetrics {
 	engine := newEngineMetrics()
 	engine.register()
 
+	epoch := prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: namespace,
+		Subsystem: innerRingSubsystem,
+		Name:      "epoch",
+		Help:      "Current epoch as seen by inner-ring node.",
+	})
+	prometheus.MustRegister(epoch)
+
 	return &StorageMetrics{
 		objectServiceMetrics: objectService,
 		engineMetrics:        engine,
+		epoch:                epoch,
 	}
 }
+
+// SetEpoch updates epoch metric.
+func (m *StorageMetrics) SetEpoch(epoch uint64) {
+	m.epoch.Set(float64(epoch))
+}