This commit will be applied during 10.5->10.6 merging as a part of
conflicts resolution.

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SET GLOBAL innodb_monitor_disable=all;
-------------------------

was set at the the beggining of mon_lock_wait_current_count test,
because lock_row_lock_time_avg is filled under lock_sys.wait_mutex lock,
which is held by connection 2:

--------------------------
--connect (con2,localhost,root,,)
SET DEBUG_SYNC="lock_wait_before_suspend SIGNAL blocked WAIT_FOR cont";
BEGIN;
--send SELECT * FROM t FOR UPDATE
-------------------------

That is why the followin SELECT is blocked:

-----------------------
SELECT name, count FROM information_schema.innodb_metrics
  WHERE name ='lock_row_lock_current_waits';
-----------------------

And setting innodb_monitor_disable=all causes innodb monitor enabling
status changing. And the status is not restored to default value even
after
-----------------------
SET GLOBAL innodb_monitor_reset_all=default;
SET GLOBAL innodb_monitor_enable=default;
-----------------------
execution. See MDEV-32553 for details.

The fix is to disable not all the monitors at the beggining of the test,
but only the monitor, which causes hanging, i.e. lock_row_lock_time_avg.

MDEV-32553 it the root case, the current fix is just workaround.
10.5 is not affected, as there is difference in innodb.monitor.