The 'changeStreamPreImages' serverStatus section has additional statistics in a single-tenant enviornment.

numDocs: <int> // same as $collStats.count
totalBytes: <int> // same as $collStats.size
avgDocSize: <int> // same as $collStats.avgObjSize (like collstats, only included when numDocs is non-zero)
storageSize: <int> // same as $collStats.storageSize
freeStorageSize: <int> // same as $collStats.freeStorageSize

Description of Linked Ticket

As part of the technical discussion for the truncate refactor of preimages removal, we discovered that currently there's potentially no good way of detecting storage issues with preimages.

To be symmettrical with the oplog, we should explore adding $collStats for preimages in single tenant environments. That way we'll be able to more easily diagnose issues with the collection.