LU-10499 pcc: add some statistics data

LU-10499 pcc: add some statistics data

Add statictics of the number and total size of pcc attached files
and pcc hit files.

EX-bug-id: EX-4433
Change-Id: Ib0e429c636298d4c6ff06d84a416073895b86184
Signed-off-by: Lei Feng <flei@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54458
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>