My favorite measurements:
• # of products built on top of a shared platform framework (combinatorial innovation)
• developer velocity: how long did it take to build XYZ feature before the platform and after? you can get granular with these too - development time, test time, review cycle time, deployment time
• NPS score - engineers are our customers and getting a pulse from them every 3-6 months is important
• product uptime, availability, time to resolve bugs - if you’re building an infrastructure foundation, it has to have SLAs better than any of the products