Once your Messenger experience is launched, there are lots of ways to collect valuable insights and analytics on how it is being used and what people on Messenger think about it.
You can also view your Facebook page and check the Messages tab under Insights.
This is a good spot to check your spam and block rates. Keep them low by focusing on a good user experience!
You can export analytics data by navigating to App Analytics under Messenger. This is restricted to admins of the app.
You may also consider integrating your existing analytics provider with Messenger.
Most analytics providers allow you to create server side events, these can then be used to measure retention, engagement, errors and other metrics that are important to your experience.