Add exception_controller option to Rails

[delner]

If a Rails app has a custom error controller defined via config.exceptions_app= for the purpose of rendering errors, the name of this custom error controller and action will supersede the name of the controller that originally raised the exception for the trace. This is because the errors controller runs afterwards, and replaces the name on the parent span, no matter what. Ultimately, it results in all error traces being listed under "ErrorsController#internal_server_error" or similarly named resource, which isn't a great experience.

This pull request introduces the exception_class option to Rails configuration, which allows a user to specify a class or module to identify a custom exception controller with. Spans for any controller that matches this definition will not override the parent span's resource name.

If you do not specify this option, the tracer will "guess" if the controller is an error controller, by looking for the presence and value of an action_dispatch.exception header, which is typically provided to error controllers in Rails. Most users should see improved behavior without any additional configuration.

In practice:

Datadog.configure do |c|
  # Implicitly guess at exception controllers
  c.use :rails
end

OR

Datadog.configure do |c|
  # Explicitly defining an exception controller
  c.use :rails, exception_controller: ErrorsController
end

And a resulting trace with improved resource name:

[palazzem]

@delner can you also update the documentation here? https://github.com/DataDog/dd-trace-rb/blob/bc978254a6c6e1e66b48d5ca64c236693f108fa9/docs/GettingStarted.md#ruby-on-rails

I would say to not give details about the false case.

[palazzem]

one question and a nitpick, otherwise it looks good to me!

[palazzem]

it's just "cosmetic", but this is the base case because most of developers don't set exception_controller, so would be great having it as a first if condition.

[delner]

Alright, I can rearrange this.

[palazzem]

can you provide more details about this change?

[delner]

It was only passing a string of the class name before. We need the actual constant now, so we can actually do type checking.

[palazzem]

changing that doesn't have any side effect for the rest of the code, right?

[delner]

Nope, it was being used to tag the name on the span. Since "#{class}" is the same thing as class.name, it has no change.

[delner]

Feedback addressed @palazzem !

[palazzem]

Good to me! thank you very much!