how GTM (irule) check the available node number in a given LTM server pool

I need to configure a DNS intelligence to resolute a URL based on how many nodes availble in a site. if the available nodes number is less than a number, then the URL need to be resolved to the vip on other site.

my question: how GTM (irule) check the available node number in a given LTM server pool (let us assume the iquery is connected between GTM and LTM cross site).

Thanks a lot.