Crisper examples of runtime metrics.

[jswrenn]

For each task metric, it's fairly easy to write a crisp, self-contained example that reliably induces a change in a metric. For runtime metrics, it's currently not so easy to do this, because:

1. runtime metrics are buffered
2. some runtime metrics are dependent on scheduling pathologies that are finicky to induce

We could resolve the first obstacle to provide some mechanism to flush metrics on-demand. For the second obstacle, I'm not sure there's much we can do.