How do I add include path for header only libraries installed from github?

Im trying to install this header only library: GitHub - biaks/prometheus-cpp-lite: C++ header-only prometheus client library for quickly and easily adding metric (and profiling) functionality to C++ projects.

In my flake.nix I have:
prometheus.url = “github:biaks/prometheus-cpp-lite”;
prometheus.flake = false;

When I run nix develop, I see prometheus in the /nix/store.
However, it’s not part of CMAKE_INCLUDE_PATH. How do I add it?