Add security update metrics for yum.sh

[Britaliope]

Small modification to yum.sh to integrate new gauges that only list security updates.

On older systems, it require yum-plugin-security to work, but on newer systems (el7 and newer IIRC) it is native.

sample output:

# HELP yum_upgrades_pending Yum package pending updates by origin.
# TYPE yum_upgrades_pending gauge
yum_upgrades_pending{origin="appstream",category="all"} 87
yum_upgrades_pending{origin="baseos",category="all"} 192
yum_upgrades_pending{origin="crb",category="all"} 1
yum_upgrades_pending{origin="epel",category="all"} 43
yum_upgrades_pending{origin="extras",category="all"} 1
yum_upgrades_pending{origin="appstream",category="security"} 22
yum_upgrades_pending{origin="baseos",category="security"} 42