stat: Print number of samples in bw and iops stats

This adds the actual number of samples for bw and iops stats printed
in normal and json fio output. Example for normal output:

   bw (  MiB/s): min=  120, max= 3304, per=0.10%, avg=2659.34, stdev=1085.25, samples=  109
   iops        : min=102776, max=830416, avg=675639.05, stdev=280619.99, samples=   21

Note: This example was created using options
"--write_bw_log=... --log_avg_msec=100 --runtime=11s" (ie. no iops log
written). That is why number of samples differs for IOPs and BW.

Signed-off-by: Andreas Herrmann <aherrmann@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>