The actual metrics (not necessarily easily quantifiable) are the desired traits you put in your job description; they don't correlate perfectly.
I very intentionally did not write anything about finding engineers who are just good at the things you care about and not at other stuff, because every bit of data I have says there is a considerable component of general engineering skill underlying most eng roles. No, it isn't totally one dimensional, but (in a principal-component-analysis sense) it is fairly low-dimensional.
There really are just better and worse engineers in the sense that eng A is better than eng B for virtually every job. But that's precisely why recognizing the competitiveness of hiring is important - the more you insist on narrowing your pool, especially in ways others also narrow theirs, the less likely you are to find the rare unknown great engineer.
Still, when we're staffing, there's a world of difference between the great engineer who is happy being mostly left alone and writing complex but well-specced SQL queries for 12 weeks and the great engineer who can balance software architecture, customer meetings, and programming for the same project.