Add :formatter to ExUnit.CaptureLog options

[halostatue]

Mostly modified :log_capture_on handler for ExUnit.CaptureServer to respect a :formatter option instead Logger.default_formatter/1.

Updates were made to documentation and tests to reflect this change.

[halostatue]

This is a minimal adaptation of the changes I made when publishing the proof-of-concept / backport package that I created for this, CaptureLogger. In CaptureLogger I'm testing with LoggerJSON, but I've created a much more simplistic "ReverseLogger" that stringifies and reverses the message being logged.

In CaptureLogger, I am using a default formatter possibly pulled from Application.get_env/2. It feels like we could pull from a possibly configured Application.get_env(:logger, :default_handler, %{})[:formatter] value here but that would make this a breaking change for anyone using ExUnit.CaptureLog with a configured formatter in the default handler (e.g., they're using LoggerJSON). Alternately, we could have a new config (Application.get_env(:ex_unit, :capture_log_default_formatter)). Not sure but open to suggestions.

I also think that I could duplicate one of the tests so that it shows the use of leftover options, as I'm showing two variations of {module, opts} (one of them with module.new/1). Let me know if you want that.

[whatyouhide]

Lovely

[josevalim]

💚 💙 💜 💛 ❤️

[sabiwara]

According to the spec, formatter can be {module(), term()} | nil, but wouldn't this fail in the nil case?

Should it be Keyword.get(opts, :formatter) || Logger.default_formatter(opts), or should we fix the spec?

[halostatue]

That's a good point. I can make either change.