When applying an oplog entry of fast_count_metadata_store_timestamps, it means the size and count info is correctly persisted up until the valid-as-of timestamp replicated in the oplog entry. As a result, the in-memory delta store should remove all entries with an earlier timestamp. 

This is applicable for both secondaries during steady state oplog application to maintain their in-memory stores, and the primary when applying the oplog entries from the last checkpoint during startup recovery.