Use relative path instead of absolute path in symlink_path for the out file plugin

[domna]

Which issue(s) this PR fixes:
There is no particular issue associated. If necessary I can create one.

What this PR does / why we need it:
This PR moves from an absolute link to a relative link for symlink_path parameter. Otherwise, this does not work in a dockerized environment where the path is wired to the the outside of the container.
I can also add an additional parameter to switch between absolute and relative linking if necessary and/or desired.

Docs Changes:
I think doc changes are not necassary. However, as far as I read it the documentation does not state whether its an absolute or relative link and it might be useful to add in the scope of this PR.

Release Note:
Uses relative link for symlink_path in the out file plugin.

[Watson1978]

It might be better to keep the default behavior.
If you want to use relative paths, please use the option function.

[daipom]

Thanks for this PR!

This PR moves from an absolute link to a relative link for symlink_path parameter. Otherwise, this does not work in a dockerized environment where the path is wired to the the outside of the container.

Could you please explain in more detail?
How do you use this relative simlink?

[domna]

Could you please explain in more detail?
How do you use this relative simlink?

Sure. In my project I'm using fluentd to collect the logs in a docker compose stack. For some containers I want to write file based logs to disk, i.e., I mount them from the external fs into the container. I use the symlink_path to simulate some sort of log rotate pattern (where I have one fixed file keeping the latest logs), which I need for the application running on the bare metal system.
Now, using the symlink_path (as implemented before) works inside the fluentd container but it breaks on the outside, because the absolute path of the directory is obviously different.
I hope this makes it a bit clearer...

It might be better to keep the default behavior.
If you want to use relative paths, please use the option function.

You mean keeping the behaviour of symlink_path as an absolute link and adding an additional option, e.g., relative_symlink_path = true/false to switch to relative links? I can certainly implement it like this.