Add unit tests for core

**Feature request**
It would be great to have monitoring for middleware.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_monitoring` with default `false`.

The issue names no files, tests, or entry points. First locate the core middleware and configuration handling, then determine how monitoring should be exposed for production use. Done means the new enable_monitoring option defaults to false, its behavior is covered by unit tests, and the middleware monitoring requirement is defined.

Written by the indexing model from the issue text.