code: func (s *State) InvokeGlobalCallbacksOnFlush() {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  
  	s.invokeGlobalCallbacksOnFlush = true
  }
docs: // InvokeGlobalCallbacksOnFlush signals that the registered global callbacks
  // are to be be executed upon flushing registered metrics.