Merge rack and rails services

[p-lambert]

This PR puts rack and rails-controller spans under the same service name by default. For distinct service names, use the controller_service option as it follows:

Datadog.configure do |c|
  c.use :rails, service_name: 'rack-handler', controller_service: 'rails-controller'
end

Breaking change

It's worth mentioning this is a breaking change since it will affect metrics associated with rails controllers and rack middleware, so make sure to update any monitors that rely on these services, or use the controller_service option to have the same name you're using now.

[palazzem]

Good to go! thanks for this update.