History List Length
History list length：当前事务历史列表的长度。如果 undolog 没有 purge 时，该指标就会不断增加。这个时候，一般是某个长事务一直在运行，导致后面的 undolog 没办法 purge。
该数值似乎只能通过 show engine innodb status 来获取到。
MySQL> show engine innodb status\G
*************************** 1. row ***************************
Type: InnoDB
Name:
Status:
=====================================
2022-05-27 00:01:46 139760858244672 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 43 seconds
-----------------
BACKGROUND THREAD
-----------------
srv_master_thread loops: 4146 srv_active, 0 srv_shutdown, 76427 srv_idle
srv_master_thread log flush and writes: 0
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 5954
OS WAIT ARRAY INFO: signal count 60629
RW-shared spins 0, rounds 0, OS waits 0
RW-excl spins 0, rounds 0, OS waits 0
RW-sx spins 0, rounds 0, OS waits 0
Spin rounds per wait: 0.00 RW-shared, 0.00 RW-excl, 0.00 RW-sx
------------
TRANSACTIONS
------------
Trx id counter 903438
Purge done for trx's n:o < 883049 undo n:o < 0 state: running but idle
History list length 9746