[#1054] neofs-node: add epoch metric

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

cmd/neofs-node/config.go

@@ -316,6 +316,7 @@ func initCfg(path string) *cfg {
 
 	if metricsconfig.Address(c.appCfg) != "" {
 		c.metricsCollector = metrics.NewStorageMetrics()
+		netState.metrics = c.metricsCollector
 	}
 
 	c.onShutdown(c.clientCache.CloseAll) // clean up connections

cmd/neofs-node/netmap.go

@@ -10,6 +10,7 @@ import (
 	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	nodeconfig "github.com/nspcc-dev/neofs-node/cmd/neofs-node/config/node"
 	"github.com/nspcc-dev/neofs-node/pkg/core/netmap"
+	"github.com/nspcc-dev/neofs-node/pkg/metrics"
 	"github.com/nspcc-dev/neofs-node/pkg/morph/client/netmap/wrapper"
 	"github.com/nspcc-dev/neofs-node/pkg/morph/event"
 	netmapEvent "github.com/nspcc-dev/neofs-node/pkg/morph/event/netmap"
@@ -30,6 +31,8 @@ type networkState struct {
 	controlNetStatus atomic.Value // control.NetmapStatus
 
 	nodeInfo atomic.Value // *netmapSDK.NodeInfo
+
+	metrics *metrics.StorageMetrics
 }
 
 func newNetworkState() *networkState {
@@ -44,6 +47,9 @@ func (s *networkState) CurrentEpoch() uint64 {
 
 func (s *networkState) setCurrentEpoch(v uint64) {
 	s.epoch.Store(v)
+	if s.metrics != nil {
+		s.metrics.SetEpoch(v)
+	}
 }
 
 func (s *networkState) setNodeInfo(ni *netmapSDK.NodeInfo) {

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))
+}