As part of SWATCH-3784, an attempt was made at adding this gauge metric but the 'select count' query was taking a long time to complete against a large outbox table in stage, which caused an increased DB load and instability of the SMHBI service.

The commit that added this metric can be found here, but will be reverted to stabilize the service.

This card is to improve the query that provides the count metric so that the metric can be enabled again. 

Acceptance Criteria

The performance of the query used to determine the current outbox count is improved and the metric provider is active in stage without causing issues.