level.go
func AddModuleLevel(backend Backend) LeveledBackend {
	var leveled LeveledBackend
	var ok bool
	if leveled, ok = backend.(LeveledBackend); !ok {
		leveled = &moduleLeveled{
			levels:  make(map[string]Level),
			backend: backend,
		}
	}
	return leveled
}
func AddModuleLevel(backend Backend) LeveledBackend {
	var leveled LeveledBackend
	var ok bool
	if leveled, ok = backend.(LeveledBackend); !ok {
		leveled = &moduleLeveled{
			levels:  make(map[string]Level),
			backend: backend,
		}
	}
	return leveled
}
// AddModuleLevel wraps a log backend with knobs to have different log levels
// for different modules.