How to include Innodb_trx_id in MySQL slow log

Question

Evaluating MySQL's slow query log with pt-query-digest from the Percona Toolkit, I need to group queries by transaction. As it seems, the easiest way to do this is to group by the Innodb_trx_id attribute.

However, my logs don't include this attribute. I tried with vanilla MySQL, MariaDB and Percona Server -- so I figure, you probably need to enable logging of this attribute. Any pointers on how to do this?

Thanks, Victor