An Azure service that is used to provision Windows and Linux virtual machines.
Ok I got it now. It was a little bit of a hassle because the metric "Hearbeat" is not available under standard VM metrics (waagent), but only in connection with Log Analytics (omsagent). Connect VM to Log Analytics Workspace where your create a monitoring rule with a condition that monitors the VM's Hearbeat signal. As action, create a PowerShell automation runbook which reboots the VM. Basically it's like @theodorbrander mentioned, only the Log Analytics part was missing.