With:
global index_add_counter
You are not defining, just declaring so it's like saying there is a global index_add_counter variable elsewhere, and not create a global called index_add_counter. As you name don't exists, Python is telling you it can not import that name. So you need to simply remove the global keyword and initialize your variable:
index_add_counter = 0
Now you can import it with:
from app import index_add_counter
The construction:
global index_add_counter
is used inside modules' definitions to force the interpreter to look for that name in the modules' scope, not in the definition one:
index_add_counter = 0
def test():
global index_add_counter # means: in this scope, use the global name
print(index_add_counter)