KQL for checking for checking "Could not find any Session Hosts available"

I used this in an alert rule

// Active sessions on SessionHost
// Display a graph of active sessions.
let GranularityInterval = 30m; // Time resolution for query results (min value is 30s).
WVDAgentHealthStatus // Fires every ~30s
// Ensure only one data point is provided per host in the pool
| summarize PeakSessionsByHost=max(toint(ActiveSessions)) by SessionHostName, bin(TimeGenerated, 30s), _ResourceId
// Sum up the values for all of the hosts in each pool
| summarize SessionsByHostPool=sum(PeakSessionsByHost) by TimeGenerated, _ResourceId
// Reduce time resolution to desired GranularityInterval and report the peak session count for each pool in that time window
| summarize max(SessionsByHostPool) by bin(TimeGenerated, GranularityInterval), _ResourceId
| render timechart