wrapper: use trace2 counters to collect fsync stats
As mentioned in the thread starting at [1], trace2 counters should be
used to count events instead of ad-hoc static variables.
Convert the two fsync static variables to trace2 counters, reducing the
coupling between wrapper.c and the trace2 subsystem. Adjust t/t5351 to
match the trace2 counter output format.
The counters are not per-thread because the ones being replaced also
were not.
[1] https://lore.kernel.org/git/
20230627195251.
1973421-2-calvinwan@google.com/
Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>