filters.go
func Filter(l LogBackend, r Ruler) LogBackend {
	return &filter{
		LogBackend: l,
		r:          r,
	}
}
func Filter(l LogBackend, r Ruler) LogBackend {
	return &filter{
		LogBackend: l,
		r:          r,
	}
}
// Filter creates a new filter with rules r for l backend.