Add optional key argument to add-profiling!.

This adds the option to profile a fn via a different logging key, and
not the default namespace.fn-name key.
Yes, I thought about this. I could add a `doseq` or an `are`, but then I have to handle the different fn call in the loop or the body of the `are`. Not sure if this makes the test clearer. I could also extract a fn that does the assertion, but this messes up the line numbers. Thoughts?