Go to Settings -> Monitoring console -> Settings -> Alerts Setup. Then select and configure the following:
DMC Alert - Near Critical Disk Usage
Try this search. Schedule it to run at some interval (hourly, for example). Set the alert to trigger when the number of results is not zero.
host="MONGO" sourcetype=df | multikv fields Filesystem Type Size Used Avail UsePct MountedOn | convert auto(UsePct) | where UsePct>95