Build journald subscriber only on Linux

[swsnr]

Only build tracing-journald subscriber on Linux.

Follow up from #1744 (comment)

Motivation

journald only exists on Linux, and it's client API requires Linux-only features (memfd) for large payloads, so it makes no sense to build the subscriber on other OS.

This also prevents anyone from accidentally creating a tracing-journald subscriber on other operating systems and have their traces send into nirvana by mistake.

[swsnr]

I have no Windows system to test the build on, so I've already opened this to see what Github actions has to say about the change.

[swsnr]

@hawkw Feel free to close this if you'd like to keep journald available on other systems as well.

[hawkw]

@hawkw Feel free to close this if you'd like to keep journald available on other systems as well.

I'm not opposed to making this change in a semver-incompatible release; I just want to ensure that the MSRV issue is resolved in a semver-compatible release before we make breaking changes. :)

[Ralith]

We should not make this change, because it breaks perfectly reasonable code that tries to use journald and dynamically falls back to more general solutions in its absence.

[Ralith]

This also prevents anyone from accidentally creating a tracing-journald subscriber on other operating systems and have their traces send into nirvana by mistake.

This is already impossible, as creating a journald subscriber when journald is not present will fail with an error.

[swsnr]

I guess that's a point. It didn't occur to me that anyone would want to do this, but for some kind of cross-platform service that's perhaps actually a use case.

I'll close this MR, since the original issue is fixed.