* Log to DataDog when a welcome notification is created
* Fix typo in Metrics::RecordDailyUsageWorker
* Add Metrics::RecordDailyNotificationsWorker to log notification counts to DataDog
* Add specs around DataDog logging for welcome notifications + click events
* Call Metrics::RecordDailyNotificationsWorker from within log_daily_usage_measurables task
* Use user_id instead of user, move notification title into tags