If you want to test yourself, you should import the app/ folder to a glitch project (glitch.com), pack the entities/, intents/, agent.json and packages.json into a .zip file and import it to a dialogflow project.
At the bot's page, you should go to Fulfillment and the URL you should change for the one that you've got from your glitch project (don't forget the /dialogflow at the end of the URL).
All set, if you want to test the MongoDB as well, you should create a new collection and paste your URI link at the server.js file inside the app/ folder (these should be at the Glitch).
Now you just have to make an integration with a Facebook Page (https://cloud.google.com/dialogflow/es/docs/integrations/facebook) and now you're ready to go.
Any doubts, tips or critics, please, reach me!
(soon i'll upload some screenshots of the bot)