XML format plugin concatenates attribute values from multiple sub-elements with the same name

When an XML element has multiple sub-elements of the same name, and those sub-elements have attributes, the attribute values get concatenated in a way that it's impossible to separate.

For example, start with the documentation's published "list of books" example. Add three sub-elements named "extra" to one of the books, each having two attributes (name and value). The following is excerpted from an XML that I have attached.

\
\Mark Twain\
\The Adventures of Tom Sawyer\
\FICTION\
\1876\
\
\
\
\

The output for this turns into:
+-----------------------------------------------------------------+------------+---------------------------------+-------------+------+-----------------------------------------+
| attributes | author | title | category | year | authors |
+-----------------------------------------------------------------+------------+---------------------------------+-------------+------+-----------------------------------------+
| {"extra_name":"widthheightdepth","extra_value":"6102"} | Mark Twain | The Adventures of Tom Sawyer | FICTION | 1876 | {} |

It shows only one value for the "extra_name" attributes, which is the concatenation of the names "width", "height", and "depth" into "widthheightdepth". Similarly it only shows one value for the "extra_value" attributes, which is the concatenation of the values "6" "10" and "2" into "6102". Unfortunately it's impossible to know how to separate those concatenated strings.

I would have expected to see something like one of the following for the attributes output instead, so that the different attribute values are separable:

{{"extra_name":"width","extra_value":"6"},{"extra_name":"height","extra_value":"10"},{"extra_name":"depth","extra_value":"2"}}
or
{"extra_name":["width","height","depth"],"extra_value":["6","10","2"]}

**Desktop (please complete the following information):**
- OS: Windows 10
- Browser: N/A
- Version: 1.19.0

[books-multiple-extras.xml.txt](https://github.com/apache/drill/files/7101221/books-multiple-extras.xml.txt)