Support event lookup by both UOM and metricID

As a swatch developer, I want MeteringEventFactory to support event lookup by either UOM or metricId, so that I can later migrate event handling to use metricId exclusively.

MeteringEventFactory currently uses a strategy of just "snapshot_$metricId". We'd like to move to a strategy of "snapshot_$productId_$uom".

Done:

• MeteringEventFactory attempts to find existing events by both "snapshot_$metricId" and "snapshot_$productId_$uom" (this should be done in the query to avoid introducing an extra round-trip to the DB).

To Test:

• manually create event that has the new snapshot_productid_uom type & make sure they're considered