Add a new queue utilization metric to report how full the queue is as a percentage. The queue utilization should be calculated as (CurrentLevel / MaximumLevel) * 100 where the level variables are defined by:

Acceptance Criteria:

• A queue utilization metric is available on the shipper's metrics HTTP interface under the /debug/vars path.
• A test exists proving that the queue utilization changes as expected as the queue fills up. An empty queue reports 0, a half-full queue reports 50, and a full queue reports 100.