A logging library is very, very far from a Turing complete language, so no Greenspunning. (Yes, I know about that Java logger fiasco from a few years ago. Not my idea.)

I don't want logging done automatically for me, what I want is too idiosyncratic. While I will log every call on major interfaces, I do want to control exactly what is printed. Maybe some parameter values are not of interest. Maybe I want special formatting. Maybe I want the same log line to include something computed inside the function. Also, most of my logging is not on entry/exit. It's deeper down, to look at very specific things.

Look, I do not want a debugger, except for tiny programs, or debugging unit tests. In a system with lots of processes, running on lots of nodes, if a debugger is even possible to use, it is just too much of a PITA, and provides far too miniscule a view of things. I don't want to deal with running to just before the failure, repeatedly, resetting the environment on each attempt, blah, blah, blah. It's a ridiculous way to debug a large and complex system.

What a debugger can do, that is harder with logging, is to explore arbitrary code. If I chase a problem into a part of my system that doesn't have logging, okay, I add some logging, and keep it there. That's a good investment in the future. (This new logging is probably at a detailed level like DEBUG, and therefore only used on demand. Obvious, but it seems like a necessary thing to point out in this conversation.)