code: public String getFirstKey() {
          return findInDocument(new Finder<String>() {
              @Override
              public String find(final BsonReader bsonReader) {
                  return bsonReader.readName();
              }
  
              @Override
              public String notFound() {
                  throw new NoSuchElementException();
              }
          });
      }
docs: Gets the first key in this document, or null if the document is empty.
  
  @return the first key in this document, or null if the document is empty