Add rate limiting for notifications

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

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

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

No files, tests, or entry points are named. First locate the handler and configuration entry points, then clarify whether the goal is notification rate limiting or handler monitoring; done should include the agreed behavior and an `enable_monitoring` option whose default is `false`.

Written by the indexing model from the issue text.