zinject: count matches and injections for each handler

When building tests with zinject, it can be quite difficult to work out
if you're producing the right kind of IO to match the rules you've set
up.

So, here we extend injection records to count the number of times a
handler matched the operation, and how often an error was actually
injected (ie after frequency and other exclusions are applied).

Then, display those counts in the `zinject` output.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #16938

tests/runfiles/common.run

@@ -159,7 +159,7 @@ tests = ['json_sanity']
 tags = ['functional', 'cli_root', 'json']
 
 [tests/functional/cli_root/zinject]
-tests = ['zinject_args']
+tests = ['zinject_args', 'zinject_counts']
 pre =
 post =
 tags = ['functional', 'cli_root', 'zinject']