Please add documentation for this new metric called sessionCatalogSize, which will start showing-up under the LogicalSessionCache stats.
The session catalog is an in-memory cache of the config.transactions collection on MongoD and on routers (starting in 4.2) it represents the number of cross-shard transactions, which are rooted on that MongoS. This metric represents the number of entries in the in-memory cache and is a way to monitor that sessions are reaped and not kept alive indefinitely.
The logicalSessionRecordCache section of serverStatus contains details about the entries in the logical session cache. This cache serves as a source of 'truth' for which logical sessions are considered active or not and is used to reap the SessionCatalog.
However, since the SessionCatalog is linked to the internal state of transactions - it can not always be reaped when a session expires from the logical sessions cache. Because of this, for diagnostics purposes we should have FTDC data for what's the number of sessions in the SessionCatalog.
This ticket is to add a logicalSessionRecordCache.sessionCatalogSize metric, which tracks the number of the SessionCatalog entries.