redis storage for counters

gistfile1.txt

register a new counter:
metrics.init('listenbrainz', 'listenbrainz-spotify-reader', 'lemmy')
first argument is for grouping all LB containers
second argument comes from CONTAINER_NAME
third argument comes from hostname

Mappings, stored in redis when .init() is called:
service-to-container set for 'listenbrainz': (listenbrainz-spotify-reader, listenbrainz-web, listenbrainz-cron)
mapping of container to hostname: {listenbrainz-spotify-reader: lemmy, listenbrainz-web: paco}

counters - normal interaction with counters:
redis hash: listenbrainz-spotify-reader: num_users=10, num_listens=3000
redis hash: mbid_matching: artists=20, recordings=30

endpoint:
This happens on the web container, so we can't generate any data on demand
(e.g. we don't know the hostname that listenbrainz-spotify-reader is running on at this point, we need it to be stored earlier)
"get me all counters for service 'listenbrainz'"
 - looks in service-to-container set => (listenbrainz-spotify-reader, listenbrainz-web, listenbrainz-cron)
 - for each container:
    - get hostname from container-to-hostname map
    - get counts
    - build results dict
 - return list of all container dicts