Add method to get metrics from PowerSupply

[smiller248]

It appears that there is both a PowerSupply and a PowerSupplyUnit type, which both represent the same Redfish "PowerSupply" resource.

Unfortunately, PowerSupply and PowerSuppyUnit each have some features that the other does not. For my current use case, I need some fields that only exist on PowerSupply but also require the Metrics() method, which is only available on PowerSupplyUnit.

I have opted for that path of least resistance here: just adding a link to PowerSupplyUnitMetrics from PowerSupply (since it's the same underlying Redfish resource schemas, everything parses correctly). Note: there is no PowerSupplyMetrics.

Please let me know if:

1. There is a good reason that PowerSupply and PowerSupplyUnit are distinct, which am oblivious of
2. You would prefer a more comprehensive fix (probably with breaking changes) rather than the path-of-least-resistance option

[stmcginnis]

Looks like I didn't review closely enough when PowerSupplyUnit was added. That is indeed just a PowerSupply.

We should just collapse its content into PowerSupply and get rid of it. That may break some folks, but it makes sense to do. And since Gofish is still a v0.x release, I don't feel too badly about it.

This is good for now. If you'd like to follow up with more, that would be awesome. Otherwise I'll put it on my list to try to get to some day.

Thanks!