[#1658] node: Support object counter metrics

Increment shard's object counter on successful `Put` calls and decrement on
`Delete`.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/local_object_storage/engine/shards.go

@@ -17,6 +17,23 @@ var errShardNotFound = errors.New("shard not found")
 
 type hashedShard shardWrapper
 
+type metricsWithID struct {
+	id string
+	mw MetricRegister
+}
+
+func (m metricsWithID) AddToObjectCounter(delta int) {
+	m.mw.AddToObjectCounter(m.id, delta)
+}
+
+func (m metricsWithID) IncObjectCounter() {
+	m.mw.AddToObjectCounter(m.id, +1)
+}
+
+func (m metricsWithID) DecObjectCounter() {
+	m.mw.AddToObjectCounter(m.id, -1)
+}
+
 // AddShard adds a new shard to the storage engine.
 //
 // Returns any error encountered that did not allow adding a shard.
@@ -35,6 +52,15 @@ func (e *StorageEngine) AddShard(opts ...shard.Option) (*shard.ID, error) {
 		return nil, fmt.Errorf("could not generate shard ID: %w", err)
 	}
 
+	if e.metrics != nil {
+		opts = append(opts, shard.WithMetricsWriter(
+			metricsWithID{
+				id: id.String(),
+				mw: e.metrics,
+			},
+		))
+	}
+
 	sh := shard.New(append(opts,
 		shard.WithID(id),
 		shard.WithExpiredTombstonesCallback(e.processExpiredTombstones),