LU-17005 obdclass: allow stats header to be disabled

LU-17005 obdclass: allow stats header to be disabled

Add a global "enable_stats_header" tunable parameter that can be
set to enable/disable the "start_time" and "elapsed_time" fields
in the standard lprocfs "stats" files.

Default to enabled, since this landed shortly after v2_14_0.

Test-Parameters: trivial
Fixes: 5efb892396e3 ("LU-11407 obdclass: add start time to stats files")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I460b957447bfb83e6d4fd7395b79ce994f3ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51823
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>