feat: Sinatra Rack Events

This PR makes it possible for users to toggle between using the Rack Events and Middleware based instrumentation

See open-telemetry#341
Depends on open-telemetry#342

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

@@ -28,7 +28,7 @@ def render(_engine, data, *)
                 end
               end
             end
-            app.use OpenTelemetry::Instrumentation::Rack::Middlewares::TracerMiddleware
+            app.use(*OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.middleware_args)
             app.use Middlewares::TracerMiddleware
           end
         end