SHOW ENGINE INNODB STATUS and SHOW GLOBAL VARIABLES were blocking on the locks used
to access the history length in MDEV-29141. While the reason for the blockage
was elsewhere, we should make these monitoring commands less blocking as there
is a trx_sys.history_size_approx function that can be used.

SHOW ENGINE INNODB STATUS and SHOW GLOBAL STATUS LIKE
'innodb_history_list_length' and Innodb Monitors can use
trx_sys.history_size_approx().