How to manage Log file for each tenant?
It sounds that you're using the Subscription edition, multi-tenancy is not available on the Community Edition. The members of this forum are expert on the Community edition. Therefore I would recommend you to get in touch with your point of contact in Bonitasoft to find a personalized solution.
That being said, I think you're right the custom appender that would filter the logs based on the Tenant_ID seem to be the best solution. Not sure the tenant id is present for all the logs though.