Provide some statistics about asynchronous IO reads and writes:
 - number of pending operations
 - number of completion callbacks that are currently being executed
 - number of completion callbacks that are currently queued
   (due to restriction on number of IO threads)
 - total number of IOs finished
 - total time to wait for free IO slot
 - total number of completions that were queued.

Also revert tpool InnoDB perfschema instrumentation (MDEV-31048)
That instrumentation of cache mutex did not bring any revelation (
the mutex is taken for a couple of instructions), and made it impossible
to use tpool outside of the server (e.g in mariadbimport/dump)