Fix Rack middleware name patching

[delner]

Spans for Rack are sometimes getting bad resource names like #GET when middleware_names is enabled and, somehow, the Rack middleware doesn't get properly patched.

This pull request changes a few things in an attempt to address this:

1. Prevents the Rack patcher from prematurely initializing the Rails middleware stack, which could cause serious problems. (e.g. missing middleware if added via initializer.)
2. Adds middleware_names option to Rails, so Rails can properly patch Rack after the Rails application is fully initialized, when the middleware will no longer change. This change also allows users to remove use :rack if this was the only option they needed it for.
3. If middleware_names is enabled, and a good resource name isn't available, it reverts to the old behavior instead of printing a bad resource.
4. Only patch Rack middleware if the middleware_names option is true. So that if use :rack is called before the Rails application is finished initializing, it allows a subsequent use :rack, middleware_names: true call to do the actual patching at the correct time.
5. If you want to use middleware_names with use :rack, you must also provide application. Helps prevent the Rack patcher from causing the bug in point 1. (Breaking change for anyone using use :rack, middleware_names. They'll need to add application, otherwise the option will have no effect.)

[palazzem]

This improvement looks good to me. Let's keep it on-hold so that we can test it properly and provide the deprecation warning.

[palazzem]

That sounds great, maybe we should add a deprecation warning with a condition such as:

if get_option(:middleware_names) && !get_option(:application)
  log_deprecation_warning('you should pass the application or use Rails')  # change it with a proper message
end

[palazzem]

👍

[palazzem]

since it's already initialized, this one works great

[palazzem]

let's keep these functionalities disabled by default; we'll revisit it before going 1.0.