You can assign a value to each log message using something like a scripted field if they follow some kind of pattern and then do a histogram to chart the values of those fields per the interval you receive the log messages(if you receive messages consistently that say the status of the server).
if not, then I just recommend setting up Heartbeat and then you have a specific app in Kibana (Uptime) that will do everything for you and where you can easily setup alerts.