default_context.go
func (d *DefaultContext) LogField(key string, value interface{}) {
	d.logger = d.logger.WithField(key, value)
}
func (d *DefaultContext) LogField(key string, value interface{}) {
	d.logger = d.logger.WithField(key, value)
}
// LogField adds the key/value pair onto the Logger to be printed out
// as part of the request logging. This allows you to easily add things
// like metrics (think DB times) to your request.