Some of custom performance counter keys requires datastore instance parameter, for example vmware.vm.perfcounter[{$URL},{HOST.HOST}, "datastore/read[average]"] does not have aggregate values and requires explicit instance parameter.

But currently there are no way to tell the datastore instance name in Zabbix - so we need a key to expose it: vmware.hv.datastore.instance[<url>,<uuid>,<datastore>]

And a prototype item based on this key should be added to datastore discovery in VMware guest template.

Also add some additional logging with trace log level to print out the retrieved instances during performance counter update.