Improve rack resource names

[p-lambert]

This PR is an attempt to provide better resource names for rack.

Currently, rack resource names are <REQUEST_METHOD> <RESPONSE_STATUS>.

We're providing an alternative that should generate more meaningful names. The approach taken here is to return the name of the middleware that effectively generated the response.

Let's say you have an AuthenticationMiddleware. In a Rails application context, an unauthorized request would never hit the Rails stack and it would surface as something like aGET 401 resource name.
By enabling the middleware_names option will you get a AuthenticationMiddleware#GET resource name instead.

Datadog.configure do |c|
  c.use :rack, middleware_names: true
end

Also worth noting here that in case the HTTP request ever "hits" your Sinatra/Rails routing layer you will still get the resource names provided by them (e.g, MailerController#create)

[delner]

Minor, non-blocking thing: would it be cleaner to do a simple if else instead of a return? Usually I see return only if you need to short-circuit a lot of code quickly (Opting not to use return is usually out of the general concern of return's side effects.)

[p-lambert]

Sure, I'll make that change 👍

[delner]

Not sure I follow this... "Return patched if patched defined, otherwise return... patched"? Redundant?

[p-lambert]

This is for having a memoized value that can be false.

Ideally we should have:

def patched?
  @patched ||= false
end

But that obviously doesn't work for falsey values.

[delner]

👍