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>

wrapper.h

@@ -87,11 +87,6 @@ enum fsync_action {
  */
 int git_fsync(int fd, enum fsync_action action);
 
-/*
- * Writes out trace statistics for fsync using the trace2 API.
- */
-void trace_git_fsync_stats(void);
-
 /*
  * Preserves errno, prints a message, but gives no warning for ENOENT.
  * Returns 0 on success, which includes trying to unlink an object that does