Telemetry is one of the additional three factors introduced in Beyond the Twelve-Factor App. Developing your application is only the first aspect of a cloud native developer’s job. Feedback from real users hitting your app deployed in production is where you can learn and improve your app.
When you add these to your Spring Boot app, you get automatic embedded monitoring and reporting in the application, along with many extension points for adding custom health indicators and other metrics. Read the Spring Actuator guide to build a RESTful web service, or add actuators to one of the applications you have already crafted as part of this guide.