multi.go
func (b *multiLogger) SetLevel(level Level, module string) {
	for _, backend := range b.backends {
		backend.SetLevel(level, module)
	}
}
func (b *multiLogger) SetLevel(level Level, module string) {
	for _, backend := range b.backends {
		backend.SetLevel(level, module)
	}
}
// SetLevel propagates the same level to all backends.