Logging documentation should explain InitializeLog and SetStderrThreshold

[pjh]

Describe the issue

I developed a C++ service that uses Abseil logging and I could not figure out why my log messages with INFO severity did not appear in the Linux systemd journal. I reviewed https://abseil.io/docs/cpp/guides/logging and its information about log severity, and I tried defining ABSL_MIN_LOG_LEVEL to 0, but it did not help.

I had previously added a call to absl::InitializeLog() (which is not explained in that docs page) in order to eliminate this warning from my server's output: WARNING: All log messages before absl::InitializeLog() is called are written to STDERR. I eventually figured out to my dismay that removing this call to InitializeLog() was all I needed to fix my problem: INFO log messages now appeared in the journal output.

I eventually stumbled upon an example that showed me how to properly initialize Abseil logging by calling absl::SetStderrThreshold() in addition to InitializeLog(). That led me to globals.h which also explains that --minloglevel can be used to control this behavior.

I feel misled since Abseil told me to call InitializeLog(), but it didn't tell me that I might also need to call SetStderrThreshold(). I did the right thing and read the documentation, but none of InitializeLog(), SetStderrThreshold() or --minloglevel are mentioned at all at https://abseil.io/docs/cpp/guides/logging. It would be great if this documentation could be improved, it would have saved me quite a bit of troubleshooting.

Steps to reproduce the problem

Write a C++ service with a main that begins with:

int main(int argc, char **argv) {
  absl::InitializeLog();
  absl::ParseCommandLine(argc, argv);

Add some LOG(INFO) and LOG(ERROR) messages in the service. Build it and register the service with systemd on a Linux system. Start the service then run journalctl -u <service-name>. Observe that the log messages with INFO severity do not appear in the journal, but the messages with ERROR severity do.

Add a call to absl::SetStderrThreshold(absl::LogSeverityAtLeast::kInfo); at the start of main(), rebuild and restart the service and observe that now INFO log messages appear in the journal.

What version of Abseil are you using?

https://github.com/abseil/abseil-cpp/archive/1adf896ec842bd9788a1bbede94a33e1402b8ecb.tar.gz

What operating system and version are you using?

Linux ARM64

What compiler and version are you using?

gcc

What build system are you using?

Bazel 6.0.0

Additional context

No response