WHAT

Update the Monitoring metrics guide to describe the new partition size metrics that will be exposed through the Kafka Management API. That Engineering work is being tracked in MGDSTRM-9838.

WHY

The new partition size metrics underpin some new charts in the OpenShift Streams console that will enable users to see the biggest partitions per broker and per topic. See https://miro.com/app/board/uXjVPStCOjM=/?moveToWidget=3458764534853086566&cot=14. Those UI changes don't require any documentation updates (see MGDSTRM-9675), but the new metrics need to be documented in the Supported metrics section of the guide.