Add List Size Stats For All Collections as a mongodb til

mongodb/list-size-stats-for-all-collections.md

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+# List Size Stats For All Collections
+
+In [Get Size Stats for a Collection](get-size-stats-for-a-collection.md), we
+saw how to use `db.collection.stats()` and its `scale` parameter to get a
+useful size metric for a given collection.
+
+We can combine some of this concepts with some scripting to list human-readable
+size metrics for all of our database's collections:
+
+```javascript
+> db.getCollectionNames().forEach(function (collectionName) {
+    sizeInMb = db[collectionName].stats({ scale: 1024 * 1024 }).size;
+    print(collectionName + ": " + sizeInMb + "Mb");
+  })
+books: 10Mb
+authors: 2Mb
+genres: 1Mb
+```
+
+This snippet gets all the collections for the current database and iterates
+over them. For each collection name, it looks up the `size` stat for that
+collection scaled to megabytes and then prints it all out with some contextual
+information.
+
+[source](https://docs.mongodb.com/manual/faq/storage/#data-storage-diagnostics)