> I'd also look at what problems need solving, in case they can be solved without function tracing.

Without speaking to the specifics of xray, which I'm not so familiar with, I'll say that the folks who developed xray are actually part of a debugging team. They are some of the best debuggers at Google, finding incredibly complex (and, usually rarely-occurring) issues that span many layers of Google's technology stack.

So, if they decided they needed function-level tracing, as opposed to any of the other tools they have available, there's probably a good reason.