Track a 'more' metric for the Updater

The Updater logs when it has more work to do after updating a test group (with 'more = true' in the log line), but it'd be nice to have a metric that allows us to track if there are spikes in groups that don't update, or a consistent line of groups that can't catch up on repeated updates. (This is expected in small numbers because some group updates, especially for new groups, take a long time, but it's a bad sign if this climbs or stays high).