Admittedly I'm not a huge fan of having:

  slog.Info("hello, world", "user", os.Getenv("USER"))

It's a little magical that "user" is a key. So what if you have multiple key-value pairs? Arguably it most likely going to be obvious which is the keys, but having every other value be a key and the rest values seems a little clumsy.

I really like Pythons approach where you can have user="value" it makes things a bit more clear.

I think Uber chose a better approach for their Go logging library called Zap [1]

    logger.Info("failed to fetch URL",
      // Structured context as strongly typed Field values.
      zap.String("url", url),
      zap.Int("attempt", 3),
      zap.Duration("backoff", time.Second),
    )

They also have zap.Error(err) which generates the "error" key as a convention.

[1] https://github.com/uber-go/zap