[#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/control.go

@@ -30,6 +30,10 @@ func (e *StorageEngine) Init() error {
 		}
 	}
 
+	if e.enableMetrics {
+		registerMetrics()
+	}
+
 	return nil
 }