Request lines for an app's error pages log the error page's path, and some errors log two request lines #28

With config.exceptions_app = routes (an app's own error pages, such as an ErrorsController), Rails' ShowExceptions renders the error page by sending the request through the routes again as GET /<status>. The error page's controller then logs a request line of its own:

• An error raised inside a controller action logs two request lines: the action's (its method and path, the error's status) and the error page's (GET /500, GET /422).
• An error raised before the action's line is logged logs only the error page's line, with the wrong method and path. For example, ActionController::BadRequest for a query Rack can't parse is raised while Rails builds the action's instrumentation payload, so the only line is GET /400 instead of the request's own method and path.

Rails keeps the original method and path in the request's action_dispatch.original_request_method and action_dispatch.original_path headers.

LogStruct should:

1. log the error page's line with the original method and path, and
2. log one request line per request: skip the error page's line when the failed action already logged one.

To reproduce: a Rails app with config.exceptions_app = routes, match "/500", to: "errors#internal_server_error", via: :all, and an action that raises, requested with action_dispatch.show_exceptions on. The log has the action's line and a second GET /500 line.