better options to name output files

**Is your feature request related to a problem? Please describe.**

I have a config like:
```
[OUTPUT]
Name file
Match *
Format plain
Path /app/logs
```

This names the file the same as the tag name. However, I'd like the output to be .log

**Describe the solution you'd like**

Fluentd has an option to use some vars in the path:
path /path/to/${tag}/${key1}/file.%Y%m%d

This makes it east to use tag or date however you like in the filename or path.

Something along these lines would be great. Fluent-bit has separate configs for path and file so obviously it might need to work a little different. Actually, don't care about the syntax so much as long as the functionality exists.

**Describe alternatives you've considered**

I do not have a good alternative yet...

**Additional context**