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?

If you have monitoring enabled you can query the monitoring data for the index store size.
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.