Best way to create a watcher that checks index store size

I'm trying to create a watcher that checks on the store size of an index pattern. I can easily see the information from Devtools by GET /index/stats/store, but how do I do this in a watcher?

I'm trying to do an http type request as referenced below, but I'm running into problems doing this. Is this the only way to do it?

https://www.elastic.co/guide/en/elastic-stack-overview/7.2/input-http.html

If you have monitoring enabled you can query the monitoring data for the index store size.
https://www.elastic.co/guide/en/elasticsearch/reference/master/configuring-monitoring.html
The data pertaining to ES is usually stored in .monitoring-es indices afair.

I'm looking all over our .monitor* indices but cannot seem to find this information in Discover. I'm searching for the _index field equaling the index I'm concerned about but it's not showing up, only the .monitor ones are.

I can confirm that our xpack.monitoring.collection.enabled is true.