After a stream processor stop, we want Agent to scrape and publish it's final metrics to MaaS. this is especially important for the per input<>output event count stats(), which are not persisted in checkpoints.

We want to do two things:

1. During stream processor stop, Immediately close any Summary windows https://github.com/10gen/mongo/blob/master/src/mongo/db/modules/enterprise/src/streams/exec/stream_stats.h#L109. This is an engine change.
2. Ensure Agent has a chance to call getStats() before the processor finishes shutting down. This is tricky-- it might require us to switch to Agent driven shutdown logic.