Thanks for asking question and reaching here! If I am understanding right you are looking for consumed storage alerts, if so, as its mentioned in Monitoring Azure cognitive search alerts document that:
"Alerts related to consumed storage are not currently available (storage consumption is not aggregated or logged into the AzureMetrics table).
To get storage alerts, you would need to build a custom solution that emits resource-related notifications, where your code checks for storage size and handles the response."
Hope this helps, Let us know if further query or issue remains.