You can use regex to control log messages output.
Solution:
1.Add variable for get only first message
<variable name="truncated_message" value="${replace:replaceWith=...:regex=true:inner=${message}:searchFor=(\,.*$)}"/>
2.Add new variable to log layout
layout="${time}${level}${truncated_message}
You will get log like this
I hope this information will be useful to you