The command recommended by the docs to view all metrics in all indexes is:
| mcatalog values(metric_name)
But with Splunk Enterprise 7.1.2 and the Add-On for Microsoft Windows, this shows nothing.
I guess a larger question is: Is anyone using metric indexes for anything? Most of the apps I've seen have agents that produce EVENTS that contain numeric values in fields, and they then use these events for their calculations.