LU-14139 statahead: add stats for batch RPC requests

LU-14139 statahead: add stats for batch RPC requests

This patch adds stats for batch PtlRPC request. It can show the
statistical information such as how many subreqs in a batch RPC.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I2f71ff5d01ab1070bd8d771a72edd786ad27f03c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/40943
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mikhail Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>