The more I manage people (especially developers) the more I'm drawn to the adage of 'a metric that becomes a target ceases to be a useful metric' but at the same time many companies put great stock in trying to develop measurable KPIs - something that I've greatly struggled with through several roles as a development manager.
If I was going to land on anything currently it would probably be something like 'Features released within a iteratively improved estimated timeframe'. In other words a metric that focused on an improvement between now the and last time the measurement was made but not towards a specific target. Also a metric that made people realise that the focus is more around them working as a team and even across multiple teams to achieve a goal.