The investigation in a recent support ticket identified non-determinism in billing metrics for a retryable findAndModify command. A retryable findAndModify operation will store a copy of the preimage in order to support transparent retries. The operation will perform an upsert in config.image_collection, keyed by the lsid. The current working theory is that something about the conditions in the test is occasionally triggering upsert, which requires an additional read which we are not filtering out of billing metrics. 

In this ticket, we should try develop a local reproducer so that we can improve our confidence in SERVER-73666 as a solution.