Do not waste your time Googling on how to create a 2D measurement annotation in iText or any kind of measurement for that matter because it is not supported.
Yet. Hopefully soon the people behind iText will have this feature soon.
The only solution for this would be coding it in a low level approach. Meaning, you will have to look up Adobe’s documentation on PDF and add the necessary properties in iText’s dictionary to be able to simulate a distance measurement annotation.