Fix `promoted_issues` usage ping counter

As noted in #251010 (comment 423355086), the promoted_issues counter is failing on gitlab.com.

This regression was introduced by !43768 (merged), which calls snowplow_event_counts with time_period: last_28_days_time_period(column: :collector_tstamp). The snowplow_event_counts method does not expect the time_period key, and it puts that into a where clause.

To fix this, we can call snowplow_event_counts with last_28_days_time_period(column: :collector_tstamp). A test should also be added to system_usage_data_monthly that covers this.