log/filtered/filtered.go
func (f *Filtered) WithField(k string, v interface{}) log.Interface {
	val := v

	// apply the filters
	for _, filter := range f.filters {
		val = filter.Filter(k, val)
	}

	return &Filtered{
		Interface: f.Interface.WithField(k, val),
		filters:   f.filters,
	}
}
func (f *Filtered) WithField(k string, v interface{}) log.Interface {
	val := v

	// apply the filters
	for _, filter := range f.filters {
		val = filter.Filter(k, val)
	}

	return &Filtered{
		Interface: f.Interface.WithField(k, val),
		filters:   f.filters,
	}
}
// WithField filters the field and passes it on to the wrapped loggers WithField