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
}
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]
SetCounterFunc set counter to a function [ called at flush interval ]