Add support for private performance metrics

[Cyberbeni]

Xcode supports more stuff to performance test than just "wall clock time", it would be cool to also see those in the formatted output

https://indiestack.com/2018/02/xcodes-secret-performance-tests/

[thii]

Did you not see the formatted output for performance tests? If not, it could be a bug.

[Cyberbeni]

There are different measurements, like Persistent VM/Heap allocations, the formatted output only contains time based measurements.

[thii]

I’d love to see a PR :)