Track fetcher task success and failures

In MESOS-7524, we added fetcher metrics for total task fetches and failed task fetches. For consistency with the similar metrics in MESOS-7842, we should switch these to track the successful task fetches and the failed task fetches. Operators can derive the total by adding the succeeded and failed counts.

ie. replace containerizer/fetcher/task_fetches_total with containerizer/fetcher/task_fetches_succeeded.