title: Track how long a thread spent actually working on a future
body: <p>I would like to keep track of how long a thread in a <code>ThreadPoolExecutor</code> spend working on a piece of code that I submitted to the pool, not how long the work item spent sitting in the pool. I, of course, call <code>future.result()</code> to get the result, but I was hoping there was some way I could call <code>future.time()</code> or something like that to get the execution time. Any suggestions?</p>