The FSM workload agg_match.js verifies the results of the aggregation by [checking the number of documents in the output collection https://github.com/mongodb/mongo/blob/72ac0d6a11f52f91736840d22b797f9cdd8d2d7f/jstests/concurrency/fsm_workloads/agg_match.js#L24. It is expecting the output collection to contain half as many documents as the source collection, but we sometimes see failures in our CI system, especially in the concurrency suites, where the output collection document count is reported as zero.

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OLD TITLE: fsm_workloads /agg_match.js fails due to fast count

The FSM workload agg_match.js uses fast count to check the collection. This is not as robust as using aggregation itself (especially given this test is in the aggregation test suite) to retrieve the collection count (using db.collection.countDocuments()). It would also be less confusing for build failures in the aggregation test suites to be failing due to fast count inaccuracies.