db_counts.js

db.adminCommand( { listDatabases: 1 } ).databases.forEach(function (dbObj) {
    var dbName = dbObj["name"];
	if (dbName != null) {
		var collectionNames = db.getSiblingDB(dbName).getCollectionNames();
		collectionNames.forEach(function (collName) {
		    if (collName != null) {
			    var count = db.getSiblingDB(dbName).getCollection(collName).count();
				print(`${dbName}.${collName}: ${count}`)
			}
		});
	}
});