Implemented sending logs to systemd-journald

[yihuaf]

@utam0k mentioned a usecase where when test youki inside k8s, it is hard to track logs of each youki implementation. A workaround was using bpf tracing the syscalls. However, it would be much better to see the actual youki logs. This is where journald comes in. It is one of the common ways for linux services and application to collect logs into a central place. In this PR, we implemented a different tracing layer that sends youki logs to journald. All logs are tagged as youki and we can obtain the logs with journalctl -t youki.

Note. I am not sure how best to enable this feature. I decided to implement this as a flag --systemd-log on the command line. However, this is not part of the OCI spec, so I added as a youki extension in the struct. However, this may not be the best way.

Another way to implement this is through features.

Tag along in this PR is a small refactor of the existing logging code. This refactors the logging into using different layers where we can get ready for the opentelelmetry work.

[codecov-commenter]

Codecov Report

Merging #1975 (05305af) into main (dc5d021) will increase coverage by 0.26%.
The diff coverage is 63.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1975      +/-   ##
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+ Coverage   65.02%   65.28%   +0.26%     
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  Files         129      129              
  Lines       14731    14784      +53     
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+ Hits         9579     9652      +73     
+ Misses       5152     5132      -20     

[utam0k]

Could you write how to test?

[yihuaf]

Could you write how to test?

In one terminal, run journalctl -f -t youki. The -t means tag. We tag all the logs using youki tag, which we hardcoded in this PR. The -f means follow, so you wait on the logs as they arrive. On another terminal, run normal youki commands with --systemd-log flag, such as ./youki --debug --systemd-log create -b tutorial test. You should see logs shows up in systemd log through journalctl.

Currently, we are exposing this feature using a flag --systemd-log. We may consider exposing this as a feature flag at compile time, so for k8s testing, no runtime flag needs to be added.

Give it a try first to see how you like how this prototype works currently. Note, journalctl itself has a lot of features, so we don't do much other than shipping the log/trace to it. For example, journalctl can present the log as json format, so our implementation don't need to do it.

[utam0k]

I love this idea ❤️