Metrics
Incident cost
Incident cost is the realized engineering hours of the breaks that landed in the period. One break is a change undone by a git revert, and its hours are the working hours from the causing change to its fix, on your company's own inferred working calendar. The cost is 0 when nothing broke in the period.
These hours are counted once, inside excess complexity, and are never added on top of a total. Failure risk cost is different: it prices future breaks, and an empty period does not remove it.