[#426] cmd/neofs-node: Fix metric option in local storage

`WithMetrics` can't make nil check without reflection so we
have to explicitly check if metrics enabled outside of
engine constructor.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

cmd/neofs-node/config.go

@@ -462,10 +462,12 @@ func (c *cfg) LocalAddress() *network.Address {
 func initLocalStorage(c *cfg) {
 	initShardOptions(c)
 
-	ls := engine.New(
-		engine.WithLogger(c.log),
-		engine.WithMetrics(c.metricsCollector),
-	)
+	engineOpts := []engine.Option{engine.WithLogger(c.log)}
+	if c.metricsCollector != nil {
+		engineOpts = append(engineOpts, engine.WithMetrics(c.metricsCollector))
+	}
+
+	ls := engine.New(engineOpts...)
 
 	for _, opts := range c.cfgObject.cfgLocalStorage.shardOpts {
 		id, err := ls.AddShard(opts...)