The recipe sends chartjson "supplemental" : {"a_stdio_uri": "..."}:
https://cs.chromium.org/chromium/build/scripts/slave/results_dashboard.py?dr=C&l=256
https://cs.chromium.org/chromium/build/scripts/slave/unittests/results_dashboard_test.py?dr=C&l=83

And the dashboard prepends "a_" to everything in the supplemental dict for chartjson:
https://github.com/catapult-project/catapult/blob/master/dashboard/dashboard/add_point.py#L315

This means that we have billions of Row entities with "a_a_stdio_uri" and the dashboard backend has to cope with it. Let's change the recipe!