[#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/range.go

@@ -60,6 +60,10 @@ func (r *RngRes) Object() *object.Object {
 // Returns ErrAlreadyRemoved if requested object is inhumed.
 // Returns ErrRangeOutOfBounds if requested object range is out of bounds.
 func (e *StorageEngine) GetRange(prm *RngPrm) (*RngRes, error) {
+	if e.enableMetrics {
+		defer elapsed(rangeDuration)()
+	}
+
 	var (
 		obj   *object.Object
 		siErr *objectSDK.SplitInfoError