I suggest you to try the following query and if it works well, create a custom alert rule with the query
Go to Log Analytics Workspace > Logs > Run the following query
The following query shows the azure virtual machine with less than 100GB disk
InsightsMetrics
| where Name == "FreeSpacePercentage"
| summarize arg_max(TimeGenerated, *) by Tags
// arg_max over TimeGenerated returns the latest record
| project TimeGenerated, Computer, Val, Tags
| where Val < 100
----------
--please don't forget to upvote
and Accept as answer
if the reply is helpful--