I would say that ideally, you have a dev tool feature to make the function available, and just have a stub by default that returns a constant or a low resolutio

I would say that ideally, you have a dev tool feature to make the function available, and just have a stub by default that returns a constant or a low resolution timestamp. That way, you can still do whatever kind of custom instrumentation you want, but it's not sitting there waiting to be exploited in an end-user's browser.