vector: add afterservices option

[happysalada]

Motivation for this change

The vector topology could depend on other services. If those services are not started vector will simply fail.
My first reflex is to provide a configurable parameter for defining the services that vector will depend on.
I had an error in production with loki.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[aanderse]

Personally I feel like it is a bit "much" to add an option for every little detail and use documentation to explain to the user they should write systemd.services.vector.after = [ "loki.service" ]; instead... but if this pattern is very common and useful maybe my opinion is the minority.

Functionally speaking this looks fine, so if you are happy and don't like my suggestion to use documentation instead I would suggest you should merge this instead 👍

[happysalada]

@aanderse thanks for the comment!
I agree with you actually.

However I feel this is a footgun, or at least a nuisance. You shouldn't have to find out that your production config is wrong.
Is there somewhere where it would be appropriate to add a comment? Something in the lines of

remember that vector config defines a topology. Depending on the config you might have to adjust the `systemd.service.vector.requires`

Or something in the like.
The idea being to help people configure vector correctly out of the box.

[happysalada]

I'll close this for now.