change job lag metric to track run_at not created_at

Change-Id: I71d02bd30b65d9051ba3fca806d0f34b1a6a44e9
Reviewed-on: https://gerrit.instructure.com/44274
Reviewed-by: Ben Chobot <bench@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>