When you create a Workflow Extension this will add the option to your Workflows in-App. Along with this, the execution of the extension will also be monitorable via the Action Log within the Workflows tool in-App.
This will show what occurs when this extension is triggered including post data or errors that are encountered.
If you're looking to have monitoring for your Workflow Extension calls via the Monitoring tab, this is currently not supported and as per your original post, would recommend you create an idea for this via our Ideas Forum.