[#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/get.go

@@ -42,6 +42,10 @@ func (r *GetRes) Object() *object.Object {
 //
 // Returns ErrNotFound if requested object is missing in local storage.
 func (e *StorageEngine) Get(prm *GetPrm) (*GetRes, error) {
+	if e.enableMetrics {
+		defer elapsed(getDuration)()
+	}
+
 	var (
 		obj   *object.Object
 		siErr *objectSDK.SplitInfoError