If you are talking about the values for the custom fields the object instances have, they should come out when you read the instances or if the argument is supported (like with the project object) using
<args name="include_custom" value="true"/>
You would have to read the instances of each object separately - one object at a time.
If you are talking about the definitions of the attributes then you would have to read again each object separately.
There is no generally available read input files to read just attribute definitions or custom attribute definitions of an object.
With some fancy gel you could accomplish that.
I should just read the object and use as many arguments as I can from
<args name="singleContentType" value="object" />
<args name="no_dependencies" value="true" />
<args name=“include_dependencies” value=“false”/>
<args name="exclude_parent" value="true"/>
<args name="noparents" value="true" />
that work for the specific object to limit the size and content.pack xml file and object query
Then I should strip out of the output manually everything else except the object section and the attritubtes.