Boby,
./logs/eventing.log
The log file /opt/couchbase/var/lib/couchbase/logs/eventing.log is for system level logging keep in mind this is not the file the output of the log(…) statements in your actual Javascript code for your Eventing functions. What goes into this log file is supervisor information for the Eventing servers and house keeping information on the Function’s life cycle (not the logic of the Function). An end user can not write a message (via a Function handler) to this log file it is intended for debugging for customers that are on support contracts. I believe redaction in the context of system logs is for things like cURL credentials.
./data/@eventing/<<function_name>>.log
The log(…) statements that you as a user add to your JavaScript go to handler specific logs live in a different set of rolling log files /opt/couchbase/var/lib/couchbase/data/@eventing/<<function_name>>.log . The information that goes to these log files is solely dependent the business logic of the Function via log(…) statements put inside the individual Function’s JavaScript. These files are not uploaded to support.
Other
I’m not very clear on why you would want to “do filter and other stuff” on either log file type.