plugin to monitor the amount of visits per origin

Similar to #361 (closed), it would be nice to monitor the amount of visits that each origin gets. The query (this time on the regular SWH DB) is the following one (see migrated/migration$50):

softwareheritage=> with origin_visits as (
     select origin, count(visit) as visits
     from origin_visit
     group by origin
     )
select count(origin), visits
from origin_visits
group by visits
order by visits desc;
  count  | visits 
---------+--------
       7 |      5
    2996 |      4
 5985138 |      3
 4853579 |      2
  956040 |      1

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Migrated from T362 (view on Phabricator)