Rails 3.3.0 through 4.x define the log format in Rails::Rack::Logger#started_request_message
github.com/rails/rails/blob/v4.2.1/railties/lib/rails/rack/logger.rb#L48-L55
github.com/rails/rails/blob/v4.0.0/railties/lib/rails/rack/logger.rb#L48-L55