监控InnoDB unpurge list length新方法

[root@yejr.me]> show engine innodb status\G
...
TRANSACTIONS
------------
Trx id counter 8684590971
Purge done for trx's n:o < 8682844874 undo n:o < 0
   state: running but idle
History list length 462312
...

[root@yejr.me]> select * from information_schema.innodb_metrics
  where name = 'trx_rseg_history_len'\G
*************************** 1. row ***************************
           NAME: trx_rseg_history_len
      SUBSYSTEM: transaction
          COUNT: 5
      MAX_COUNT: 27
      MIN_COUNT: 5
      AVG_COUNT: NULL
    COUNT_RESET: 5
MAX_COUNT_RESET: 27
MIN_COUNT_RESET: 5
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-05-20 10:41:50
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1921486
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: value
        COMMENT: Length of the TRX_RSEG_HISTORY list

set global innodb_monitor_enable = trx_rseg_history_len;