How should the lifecycle of MongoClient work?

You are using MongoClient incorrectly. As indicated in the docs

It is recommended to store a MongoClient instance in a global place, either as a static variable or in an IoC container with a singleton lifetime.

So you should rework your code to create a singleton MongoClient instance and then use that throughout your program.

The reason is that MongoClient maintains a pool of open connections that can be shared and reused for best performance.

CodeHunter