Ask HN: How do you measure your level of expertise with a programming language?

I've been reading about Quantified Self (https://quantifiedself.com/) and the empirical measurement aspect of it seems like it could be a good fit for measuring level of proficiency in using a programming language while learning it.

I plan on tracking things like the learning resources I use and how much time I spend programming. But what benchmarks, milestones, or determining factors would you use in determining how "good" you are with a programming language?