Refactor the existing stats tracking and updating
code into struct latency_stats_tracker and while at it,
count lock_acquisitions only on success.
Decrement operations_currently_waiting_for_lock in the destructor
so it's always balanced with the uncoditional increment
in the ctor.
As for updating estimated_waiting_for_lock, it is always
updated in the dtor, both on success and failure since
the wait for the lock happened, whether waiting
timed out or not.
Fixes#12190
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closes#12225
bdb6550305