Monitoring is a practice that is very very important to your application, it's often a forgotten step, until something goes wrong. You should be monitoring your application in production to ensure that it's working as expected.
You should be monitoring things like response times, error rates, and resource usage. You should also be monitoring your infrastructure to ensure that it can handle the amount of traffic that the application is serving. You might find out that your environment has too much resource and you can reduce this to save money.
You can use tools like New Relic, Datadog, Prometheus, Grafana, etc to monitor your application and infrastructure.