[#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/put.go

@@ -34,6 +34,10 @@ func (p *PutPrm) WithObject(obj *object.Object) *PutPrm {
 // Returns any error encountered that
 // did not allow to completely save the object.
 func (e *StorageEngine) Put(prm *PutPrm) (*PutRes, error) {
+	if e.enableMetrics {
+		defer elapsed(putDuration)()
+	}
+
 	_, err := e.exists(prm.obj.Address()) // todo: make this check parallel
 	if err != nil {
 		return nil, err