Free disk space is less than 5% on volume?

henry75 as per my other reply, there is an example Re: an alarm when vm has not been on for 30 minutes continuously. - Microsoft Community Hub in the [Queries] list.  You can use this as the basis of your query.

// Logical disk space % below threshold: 
// Show avg % of free Logical disk space over 10 minutes. 
let _minValue = 90; // Set the minValue according to your needs
Perf
| where ObjectName == "LogicalDisk" and CounterName == "% Free Space" // the object name used in Windows records
| where TimeGenerated >= ago(30m) // choose time to observe 
| where CounterValue <= _minValue
| summarize avg(CounterValue) by bin(TimeGenerated, 10m), Computer, InstanceName