instruments.go
func (g *Gauge) Snapshot() float64 {
	u := atomic.LoadUint64(&g.value)
	return math.Float64frombits(u)
}
func (g *Gauge) Snapshot() float64 {
	u := atomic.LoadUint64(&g.value)
	return math.Float64frombits(u)
}
// Snapshot returns the current value.