[#426] engine: Support duration metrics

With `enable metrics` option, engine will collect
durations for all public methods.

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

pkg/local_object_storage/engine/engine.go

@@ -22,6 +22,8 @@ type Option func(*cfg)
 
 type cfg struct {
 	log *logger.Logger
+
+	enableMetrics bool
 }
 
 func defaultCfg() *cfg {
@@ -51,3 +53,9 @@ func WithLogger(l *logger.Logger) Option {
 		c.log = l
 	}
 }
+
+func WithMetrics(v bool) Option {
+	return func(c *cfg) {
+		c.enableMetrics = v
+	}
+}