Update stored measurements metrics reporting #763
Description
Right now, we report the number of measurements stored in redis to Elastic using an approach that's very lightweight but not 100% precise due to key eviction (one of the measurement keys may be evicted sooner than the other). With #761 this stops working entirely as measurements no longer use a predictable number of keys.
I think for Redis, we should simply go with reporting the number of keys in the DB instead of trying to calculate how many of them are measurements. We can also add a second stat now for measurements stored in Postgres, which is much easier to calculate and track.