code: func (g Gauge) SetBatchFunc(key interface{}, init func(), f func() int64) {
  	gm.Lock()
  	defer gm.Unlock()
  
  	gauges[string(g)] = f
  	if _, ok := inits[key]; !ok {
  		inits[key] = init
  	}
  }
docs: // SetBatchFunc sets the gauge's value to the lazily-called return value of the
  // given function, with an additional initializer function for a related batch
  // of gauges, all of which are keyed by an arbitrary value.