We will get back to you on how to access Metadata in API. In the meantime, you can download all Metadata in DataBank.
1. Go to http://databank.worldbank.org
2. Select the database and variables desired.
3. Click "Download" in the top right.
4. In the pop-up box, select the Metadata tab and click Download.
This will bring up all Metadata for a particular data set.