Pipeline substring function does not support selecting an output from list

[Harmless05]

Expected Behavior

Most other functions in the pipeline supports selecting a output from the list, but not substring. There should be an option to do so.

Current Behavior

Currently the substring function only supports direct string.

Possible Solution

Supporting output list for substring function.

Context

I'm trying to shorten/truncate log messages ($message.message) that are longer than 100 characters, but I'm currently unable to do so. So adding the ability to select an output from list would be useful.
I was originally trying to substring under Alerts - Custom HTTP Notification (which would make my life much easier rather than doing a whole bunch of work to create a pipeline with rules) because I'm sending the log to other sources.

Your Environment

• Graylog Version: 6.1.5+e3ae3ce
• Operating System: Debian 12