How to check innodb history list length?
I can't find documentation confirming it anywhere, but I think it's:

select count from information_schema.innodb_metrics where name = 'trx_rseg_history_len';

The closest thing to a confirmation is a bug report referring to this counter as the history list length. Also, the current, min and max values match the logs I have recorded. It would be nice if someone familiar with the MySQL source code could confirm it.