Emit task metrics on all task completions.

[gianm]

Previously, "emitTaskCompletionLogsAndMetrics" would emit the metrics task/run/time, task/success/count, and task/failed/count only for tasks that complete due to an attached runner callback (from attachCallbacks). This patch causes metrics to be emitted whenever notifyStatus successfully marks a task as completed, which covers a wider variety of scenarios.

The prior behavior missed scenarios where the shutdown API is used on a task that the runner is aware of but has not yet been added to the queue. It could happen during Overlord startup, while the queue is initializing.