feat: Make Rack install optional for sinatra

This allows for scenarios when there are multiple Rack Applications mounted in the same builder and you want to avoid installing the Rack Events middleware multiple times in the stack.

A common example is a Rails application that mounts a Sinatra App in the routes file:

```ruby

Rails.application.routes.draw do
  mount Sinatra::Application, at: '/sinatra'
end

```

This results in the Rack middleware being installed multiple times.

Once by the Rails ActionPack instrumentation and then by the Sinatra application leading to multiple Rack spans being created in the same trace.

This change allows you to avoid this by setting the install_rack option to false when using Sinatra instrumentation and allowing users to manually configure the middleware in the stack as they see fit.

instrumentation/sinatra/lib/opentelemetry/instrumentation/sinatra/extensions/tracer_extension.rb

@@ -28,8 +28,8 @@ def render(_engine, data, *)
                 end
               end
             end
-            app.use(*OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.middleware_args)
-            app.use(Middlewares::TracerMiddleware)
+
+            Instrumentation.instance.install_middleware(app)
           end
         end
       end

instrumentation/sinatra/lib/opentelemetry/instrumentation/sinatra/instrumentation.rb

@@ -12,15 +12,22 @@ module Sinatra
       # The Instrumentation class contains logic to detect and install the Sinatra
       # instrumentation
       class Instrumentation < OpenTelemetry::Instrumentation::Base
-        install do |_|
-          OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.install({})
+        install do |config|
+          OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.install({}) if config[:install_rack]
 
           ::Sinatra::Base.register Extensions::TracerExtension
         end
 
+        option :install_rack, true, validate: :boolean
+
         present do
           defined?(::Sinatra)
         end
+
+        def install_middleware(app)
+          app.use(*OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.middleware_args) if config[:install_rack]
+          app.use(Middlewares::TracerMiddleware)
+        end
       end
     end
   end