Conventional "dev" wisdom says that LOC and PR count don't matter but I think its very easy to combine this data point with the nature of a dev's work to come up with a great heuristic on productivity.

It is good for ballparking. PR count is more interesting as if it is high it means you must have a good CI CD pipeline. I worked places where there were limits on commits a day due to monolith plus master CI/CD taking an hour. You then end up thinking of strategies like "I'll do that in the afternoon and get it to that state and it gets merged at this time" and so on. Which is inefficient.