Support zabbix[vcache,cache,<parameter>] [1] an additional <parameter> = low_memory (boolean) as it's available in the stats variable for return on the internal items [2,3]. The behavior should be such that it's "true"/1 when the "value cache is fully used: please increase ValueCacheSize" warning message [4] would be logged.

Refs:

[1] https://www.zabbix.com/documentation/2.2/manual/config/items/itemtypes/internal
[2] https://github.com/zabbix/zabbix/blob/2.2.2/src/zabbix_server/poller/checks_internal.c#L490-L500
[3] https://github.com/zabbix/zabbix/blob/2.2.2/src/libs/zbxdbcache/valuecache.c#L3010
[4] https://github.com/zabbix/zabbix/blob/2.2.2/src/libs/zbxdbcache/valuecache.c#L717-L740