A user reported some issues with the resource based tuner, and I was unable to get metric info because Python (and I believe most langs) will create a TunerHolder and not a ResourceBasedTuner when using resource-based tuning, because it simplifies the code in mixed-slot-supplier usecases.

The problem is the metrics for resource based slot suppliers only work if they've been created via a ResourceBasedTuner.

The whole attach_metrics approach in general is a bit annoying. Realistically the metrics context should probably come on the contexts passed into the slot supplier methods, so that we can more readily expose it to users in the future if desired.