mgr/zabbix: Send max, min and avg PGs of OSDs to Zabbix

mgr/zabbix: Send max, min and avg PGs of OSDs to Zabbix

We already send the max, min and avg fill ratio of OSDs but
knowing the OSD with the highest amount of PGs is also useful.

This allows admins to create a trigger should it happen that there
is a OSD with too many PGs.

This could happen if a lot of OSDs fail and PGs start to move filling
up one or more OSDs with many PGs.

As PGs eat CPU and Memory people usually like to watch out for these
situations.

Signed-off-by: Wido den Hollander <wido@42on.com>