Improve remaining work metrics for deleted objects worker

What does this MR do?

Ci::DeleteObjectsWorker uses LimitedCapacityWorker for scheduling and concurrency settings. LimitedCapacityWorker reports remaining_work_count metric to Prometheus when the job completes. With remaining_capacity as an upper limit to the count we can't see if there's more work to be done than there is available capacity.

For example, with Ci::DeleteObjectsWorker running at full capacity, the metric would be always 1 because there is only one capacity slot available(the job that it's currently executing).

Related to #247103 (closed)

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Does this MR meet the acceptance criteria?

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