Barrage: Add `EnqueuedCellCount` Metric

[nbauernfeind]

As a user of a barrage who wants to gain insights from looking at metrics collected related to propagating data to gRPC clients, I would like to better understand EnqueueMillis in the context of how many cells were recorded/copied for the off-update-graph-cycle.

One trivial metric we could add that would help is an EnqueuedCellCount.