Metrics to Show Value of a CSM from Customer's Perspective

I'm curious if anyone uses metrics to measure the value of a CSM from the customer's perspective.  All metrics and data that I have seen point to the benefit of the vendor/employer of the CSM (e.g. retention, expansion, adoption, use cases).  But in a paid-for CSM model, what metrics do you use to show value and ROI from the customer's perspective?