We could introduce a new aggregation stage $allCollectionStats. This data source would present a data stream of all collection stats for all collections. 

• This stage actually exists but it is internal ($_internalAllCollectionStats).
• We should make $allCollectionStats accept, at least, same specifications as $collStats. (Done in PM-2934).

{
 $_internalAllCollectionStats: {
   stats: { /* This is the DocumentSourceCollStatsSepc object */
     latencyStats: { histograms: <boolean> },
     storageStats: { scale: <number> },
     count: {},
     queryExecStats: {}
   }
 }
}
])

* $allCollectionStats seemed like something the cloud team would be interested in for other projects also.

At the moment we make this aggregation stage public it should be documented as "New in version x.y" and notify downstream attention.