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.
Can we break up the assertions into testing custom key behaviour, and elapsed separately?

``` clj
(deftest test-measurement-key
  (testing "with no explicit key"
    )

  (testing "with a key of :my-metric"
    ))

(deftest test-measurement-of-time-elapsed
  (let [m (<!! measurements)
        elapsed (:listora.measure-test/add-two m)]
    (is (< elapsed) 0.1)
    (is (> elapsed) 0.0)))
```