Allow customization of Rails controller resource names

[delner]

You can modify the resource of a span in a controller by doing the following:

class FooController < ApplicationController
  def index
    tracer = Datadog.configuration[:rails][:tracer]
    tracer.active_span.resource = "custom-resource-name"
    # ...
  end
end

However this wasn't working because resource names were being overridden by default names when the span was completed.

This pull request allows the above to work, by not applying default resource names if a custom resource name has already been set.

Example of it working: