Poked around their site a bit, they claim to be open source but I don't see a link to a git repo anywhere. Also searched GitHub a bit but might have missed it.
Hey @daxaxelrod! I'm one of the cofounders of Lago, the lib will be opened very soon, we're in the final steps of QA, that's why. I will make sure to ping you back here when it is.
We wanted to share this post about our first-hand experience with billing a bit ahead of this.
Thanks for your patience!