When performing a metadata checkpoint,

1. Read the valid-as-of timestamp from fast_count_metadata_store_timestamps
2. Create an oplog cursor at the valid-as-of timestamp
3. Parse and aggregate the size and count deltas tracked in the oplog entries from the valid-as-of timestamp to the latest timestamp, accumulating them by collections in memory
4. Commit the accumulated deltas and the last oplog entry's timestamp to fast_count_metadata_store and fast_count_metadata_store_timestamps