LU-17005 obdclass: allow jobstats histogram to be disabled

LU-17005 obdclass: allow jobstats histogram to be disabled

Allow the jobstats IO size histogram to be disabled with the
"enable_stats_header" tunable parameter.  This maintains
compatibility with older collectd and other stats-scraping
scripts that do not understand these new fields.

Default to disabled for EXA6.2.1, since it is new for this release.

Test-Parameters: trivial
Fixes: fde40ce32c ("LU-16087 lprocfs: add histogram to stats counter")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie8bff8ef033f0b6e28c19548b7302d0913ddf905
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51966
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>