As a swatch developer, I want UOM and metricId to have the same values so that I can retire usage of UOM.

Done:

• MeteringEventFactory updated to produce eventType w/ format "snapshot_$productId_$metricId" (which is now equivalent to "snapshot_$productId_$uom").
• PrometheusMeteringController query updated to use just `snapshot_$productId_$metricId` (which is now equivalent to "snapshot_$productId_$uom") (undoing the double lookup implemented in SWATCH-1373).

Note: blocked by SWATCH-1375 landing in prod