Add queries to test that we've the number of rows we would expect in a live DB

I thought this after we're working on #118 and i created https://pad.riseup.net/p/nht-metricsdb-data-gaps. It wouldn't be a test to run in the CI but to run against to live DB and we could schedule it to run it once a day for example.I think would help us to figure out whether there's some bug parsing data into tables, instead of checking manually when we suspect there's an issue. If it sounds a good idea, i'd start adding similar queries to the pad.