text.go
func (m *CirconusMetrics) SetTextFunc(metric string, fn func() string) {
	m.tfm.Lock()
	defer m.tfm.Unlock()
	m.textFuncs[metric] = fn
}
func (m *CirconusMetrics) SetTextFunc(metric string, fn func() string) {
	m.tfm.Lock()
	defer m.tfm.Unlock()
	m.textFuncs[metric] = fn
}
// SetTextFunc sets a text metric to a function [called at flush interval]