gauge.go
func (m *CirconusMetrics) SetGaugeFunc(metric string, fn func() int64) {
	m.gfm.Lock()
	defer m.gfm.Unlock()
	m.gaugeFuncs[metric] = fn
}
func (m *CirconusMetrics) SetGaugeFunc(metric string, fn func() int64) {
	m.gfm.Lock()
	defer m.gfm.Unlock()
	m.gaugeFuncs[metric] = fn
}
// SetGaugeFunc sets a gauge to a function [called at flush interval]