Add total supply / active-stream count aggregate to the factory

[Jaydbrown]

A cheap counter maintained on create/cancel so dashboards don't have to page the whole index.

[ZacLou]

I'd like to work on this issue. I have relevant experience with this tech stack and can deliver a clean, well-tested PR. I can submit within 1-2 days.

[ZacLou]

I'd like to work on this — opened a PR at #521.