code: func RegisterGlobalCallback(f Callback, metrics ...types.Metric) {
  	RegisterGlobalCallbackIn(context.Background(), f, metrics...)
  }
docs: // RegisterGlobalCallback registers a callback function that will be run once
  // per minute on *one* instance of your application. It is supported primarily
  // on GAE.
  //
  // You must specify the list of metrics that your callback affects - these
  // metrics will be reset after flushing to ensure they are not sent by multiple
  // instances.
  //
  // RegisterGlobalCallback should be called from an init() function in your
  // module.