[cco]

Generally you find the problems in your developer experience, and use those as your metrics. Maybe it takes three PRs and an hour and fifteen minutes to deploy to prod, lower number of PRs and minutes to deploy would be your metrics.

Or maybe to introduce a new endpoint in your API takes X amount of boilerplate lines, Y files, etc and you post mortem new endpoints after your change to ensure that number is dropping.

Talk to people, find out what their problems are, quantify the problem, measure.