[APPSEC-9343] Appsec support nested apps

[GustavoCaso]

What does this PR do?

Fix #2722

I tested the code against a local Rails app that mounts a Sinatra App.

The configuration on the Rails app looks like this:

# config/initializers/datadog.rb
Datadog.configure do |c|
  c.tracing.instrument :rails, service_name: 'gustavo-nested-apps'
  c.appsec.enabled = true
  c.appsec.instrument :rails
  c.appsec.instrument :sinatra
end

# config/routes.rb

require_relative '../app/rack_apps/my_sinatra_app'

Rails.application.routes.draw do
  root "home#index"
  mount RackApps::MySinatraApp.new => '/api'
end

To ensure we are not re-executing the request middleware code multiple times with the context of nested apps, we need to make sure we bail out at the proper time.

That removes the 🐛 that some customers are experiencing, which raises Datadog::AppSec::Processor::AlreadyActiveContextError

Also, by making sure we continue the execution of the request without adding any extra calls to the WAF, we make sure to only calls the WAF the right amount of times 😄

Motivation

Additional Notes

I added a basic integration test to validate that things work as expected.

How to test the change?

[lloeki]

LGTM, just a few notes about the comment