counter.go
func (m *CirconusMetrics) SetCounterFunc(metric string, fn func() uint64) {
	m.cfm.Lock()
	defer m.cfm.Unlock()
	m.counterFuncs[metric] = fn
}
func (m *CirconusMetrics) SetCounterFunc(metric string, fn func() uint64) {
	m.cfm.Lock()
	defer m.cfm.Unlock()
	m.counterFuncs[metric] = fn
}
// SetCounterFunc set counter to a function [called at flush interval]