Consider using formatted_message in Ruby Logger instrumentation

[wsmoak]

In #983 there was a question about whether to use msg or formatted_message in the Ruby logger instrumentation.

Now that it has been released using msg, this issue is open to decide whether to change to formatted_message.

[wsmoak]

I voted to release with msg because I didn't want to delay the merge and didn't understand exactly what the difference would be.

Now that I've used it... it should definitely be formatted_message. My logs are missing lots of info should be at the beginning of the line.

[wsmoak]

In particular, the tags from Rails.logger.tagged currently do not come through in the msg while with formatted_message they do appear. We should switch.

[kaylareopelle]

Thanks for your feedback here, @wsmoak and for opening the issue and PR!