Is it possible to monitor a keyword in a certain file located on the server via OneAgent?
monitor specific file location like this "/xyz /application/jrtemp", and any anomaly ("jretemp" has the word offline) happens it will be generate a alert.
The Plugin @Mizső mentioned is good, but doesn't work for your exact use case as it can't look into the files.
But you can adapt it or write a custom plugin that scans a file and looks for your key word.
There is a general tutorial for OneAgent extensions. As they are written in python, you can use any python tutorial for the file handling and scanning parts.
The dynatrace parts should be straight forward and can be taken from the tutorial or any other plugin. You can just unzip them and have a look if you want to see how they work.