I have given up and moved to Source Mod, with which I am hoping to have more luck but I am utterly confused on how to achieve what I want with metamod.
Namely, here are the steps that I took:
1. Made a local copy of hl2sdk-csgo
2. Inside of there, I had changed a serverplugin_sample.cpp
3. Built it - I got dll
As I understood it, the linux requires .so files instead of .dll and they should be placed inside serverfiles.
I was absolutely unable to build .so file and decided to give up.
Are there any tutorials on how to setup things and build for linux? I have a feeling that I also made a mistake in working directly under hl2sdk package and instead I should've worked outside of it, but was also unsucessful in that one as well.