Thanks Arpit for the comment. Updated the patch.
I wanted to expose the total number of timeouts, so I created additional AtomicInt in the previous patch. In the 02 patch, the metric shows the current number of timed out pending replication blocks because timedOutItems is cleared in getTimedOutBlocks().
I suspect that the current number doesn't fit for us because the number is cleared very frequently (recheck interval is 3 sec by default, which is probably smaller than the interval of metrics sink). What do you think?