I'm looking to create a custom Kubernetes controller; I do mean controller in this case because I'm not looking to create a CRD, and thus, not an operator. Basically, it would be similar to the External DNS project in that it watches annotations, and takes actions based on the presence / absence of that annotation.

Is there a a simple example of something like this? Most of the resources I find are for operators and creating CRDs, but my use case just involves looking at an annotation only.