Could you show me the path example?

Sure: /tmp/test,dev.log would be a vaild file name on unix system. In windows environments, a long filename (longer than 8 chars) may also contain a comma. The tail input would interpret this a two files, /tmp/test and dev.log. With this PR you could escape this with a \ (in the config this would look like /tmp/test\\,dev.log)

Hmm... support special case is not good for me.
How about using dummy's message like approach?

For ',' or other special case, use path ["p,ath1", "path2"].
This approach keeps to support path path1,path2 string parameter with split(',').

I've added an alternative 'paths' config setting, that would allow the form of configuration that you mentioned previously.
So now it's either
path: "test.log, test2.log"
or
paths: ["test.log", "test2.log"]

The latter form can then also include commas.

Can I help somehow to get this further?