Emit metrics with task results

While some level of failing tasks is always to be expected, we want to see if there's a sudden spike, especially if it correlates with other problems such as API unavailability or system load.

WorkRequest.mark_completed and WorkRequest.mark_aborted should increment counters labelled by task type, probably task name, and (for mark_completed) result. Since these methods may be called by Celery services rather than by the server, the comment in #1195 (comment 700138) also applies here.

Once we have this, we should add graphs of task results to metrics.freexian.com.