General inquiry about how to create an alert definition to monitor logs not being ingested from endpoint syslog servers.
VMware Aria Operations for Logs 8.x
You can review the following reference on Alert Definition and Notification configuration. Alerts in VMware Aria Operations for Logs.
You can also enable Inactive Host Notification. This will ensure an alert is triggered if and when an endpoint syslog host is not responding, hence not sending logs to Aria Operations for Logs. Configure a System Notification to Report on Inactive Hosts.