It seems that the code generates an ActivitySourceWithCodePath class for each assembly, so that when you pass one through (from say a Test project), it complains that it's not assignable.
Tests\Web\Controllers\ProductsControllerFixture.cs:49 Argument type 'ActivitySourceCodeAttributes.ActivitySourceWithCodePath [Tests]' is not assignable to parameter type 'ActivitySourceCodeAttributes.ActivitySourceWithCodePath [Web]'
Admittedly, in my tests, i'm just passing new ActivitySource("Test")), so I wonder if there might be a better way?