tracing-journald: Do not connect socket

[swsnr]

Bug Report

Currently tracing-journald subscribers immediately connect to the systemd journal socket.

I believe this implies that the subscriber will permanently fail if the connection to the socket ever drops, e.g. if journald crashes. I think the subscriber should use an unconnected socket and then use send_to to specify the target socket explicitly for every message. That'd make a subscriber "survive" a journald crash, and it's also what upstream does

I can make a pull request (based on #1744) to change this.

Crates

tracing-journald

[swsnr]

/cc @Ralith

[Ralith]

SGTM!

[Ralith]

Actually, one drawback to this: if we don't connect at startup, we lose the immediate, actionable feedback on journald being missing/misconfigured/whatever. This isn't a problem for upstream because they propagate errors to the user, but a tracing subscriber can't. Surviving a journald restart is still a huge win, but we should probably at least check that /run/systemd/journal/socket exists and is writable in new.