Add script name to resource for Sinatra if available

[delner]

This pull request prepends request.script_name to Sinatra trace resource names, if available and enabled. e.g. get '/endpoint' from a Sinatra app rooted in Rack via map '/foo' would change the trace resource from /endpoint to /foo/endpoint.

This new behavior is disabled by default. It can be enabled by adding the resource_script_names flag to the configuration. e.g.

Datadog.configure do |c|
  c.use :sinatra, resource_script_names: true
end

Or via Datadog.configuration[:sinatra][:resource_script_names] = true

This flag is useful for particular cases where there are multiple Sinatra apps configured in a config.ru that have overlapping endpoints rooted at different namespaces. Take for example the following app:

config.ru:

require 'bundler'
Bundler.require

require './lib/sinatra/app'

map '/foo' do
  run FooApp
end

map '/bar' do
  run BarApp
end

lib/sinatra/app.rb:

require 'ddtrace'
require 'ddtrace/contrib/sinatra/tracer'
require 'sinatra'

require_relative 'app/foo'
require_relative 'app/bar'

lib/sinatra/app/foo.rb:

class FooApp < Sinatra::Base
  register Datadog::Contrib::Sinatra::Tracer

  get '/endpoint' do
    'Foo!'
  end
end

lib/sinatra/app/bar.rb:

class BarApp < Sinatra::Base
  register Datadog::Contrib::Sinatra::Tracer

  get '/endpoint' do
    'Bar!'
  end
end

If you were to call GET /foo/endpoint, then GET /bar/endpoint, you would hit each of the apps correctly on their respective paths. However, the two traces created from these requests would report the same resource as /endpoint in the UI, which could lead to some confusion (since they are not the same app.)

By adding request.script_name onto the front of a path (see Rack specification for more info), it allows these traces to be disambiguated as GET /foo/endpoint and GET /bar/endpoint for a less confusing user experience.

[p-lambert]

Can we remove the empty ()? 🙏

[delner]

Yes, we can!

[p-lambert]

This feature seems kind of an advanced use-case IMO. Could we prepend this conditionally, using a configuration param? I think for most users the resource name without the script name would still be a better default.

[delner]

I think that's reasonable. Any thoughts about what you'd call it? prepend_script_name_to_resource? resource_script_names? Something less wordy?

[p-lambert]

script_name_prefix?

[p-lambert]

Very good PR description, by the way :) 👏

[delner]

Okay, modified per feedback. I added a resource_script_names flag to the Sinatra tracer, which defaults to false, disabling this new behavior by default. Also added tests to cover this flag.