I saw another post https://developer.apple.com/forums/thread/652719 where it was mentioned at this point MetricKit is not meant to be enabled within the App Extension targets and it's up to the container app to receive the payloads on behalf of the extensions. But what's the guidance to use MetricKit for System Extension targets on macOS? I've tried subscribing to diagnostic reports from both a container app and its system extension on macOS, neither could receive any payload from the system extension.
(The subscriber within the container app did receive payloads for its own reports - e.g. when the container app crashed itself, so I'm somewhat sure my test code was set up properly)