search satisfaction: Add virtualpageview metrics

One potential source of search satisfaction is virtual page views. They are unlikely to be as meaningful as a click with sufficient dwell, but they do mean something. As such add three new output columns to the metrics aggregation. The first two indicate how many serps resulted in a virtual page view, and how many sessions interacted with virtual page views. The final metric indicates how many abandoned sessions were potentially satisfied by a virtual page view.

Depends on a new event being fired by the metrics collection that provides information about virtual page views.

MR Also contains two additional patches to correctness, found while working on the virtualpageview additions, that would ideally be stacked. They end up here because there is no clean way to stack these patches.

Bug: T375387