Hi Wasfi,
By default the "main" log uses the format of "bcreportermainv1" and this format is non-editable. If you have not done already, try copying this format fields and then create a new format with the extra field added. After this go to Configuration > Access Logging > Logs > General Settings [Tab]. Select log as "main" or the one you expect to have this detail added. Change the "Log Format:" to the newly created format. Start trailing this log.
*If you are using a new custom log, make sure to add policy to write to this new log file.