My two cents
When I work on my Service Design Principles Library, the most important metric for me is the number of principles. In fact, at the moment, I have a simple goal, at the end of each year, I want to have created and edited 100 principles that will then be published in a book. I feel that the total number of principles you have in your Library is a number that can be really motivating.
Outside of this metric, here a few I use:
Number of principles in each writing stage: that's especially useful to me as I'm in editing mode so that I know how many principles just have a title, how many have rough notes, etc.
Reduction of words: For each new draft, I compare the word count to the previous draft. This helps me see if I'm cutting enough shit out or if I'm too emotional with my writing.
References: Another thing I link to count is the total number of references to which I link from my Principles. This helps me realise that I'm building a knowledge base that is not only referenced in my stories, but that has links to other theories, case studies and research elements.
As my Library of Service Design Principles is built within Notion I can automatically calculate most of these metrics.
For example, in Notion, I have a view of my principles where I can then see the average word reduction for each stage.