Thinking this can be reduced to a single metric is the blight of modern software (and business in general, I think). Mapping an individual change to improved delivery speed is in the vast majority of cases an impossible task and any decent developer knows this. It's management+ that wants simple easy metrics since they lack the deep understanding required to do their job well. Software development is - despite management's hopes - not line work. It's much more akin to R&D. The line work gets eaten up by AWS.