Its a bit unclear what kind of special parameters can be used in filebeat.yml
Specificly, I'm reffering to parameters of the elasticsearch output section.
For example the index name can be set with date:
Another example I saw in some other discussion is that I can refer to fields from the log output.
My question is what else can be done with the % sign? Can I write some Go (or other language) code? For example to get a substring from a field value, and that will be part of the index name?