There isn't a way to automatically add spans for arbitrary functions in Node, no. There's currently a proposal to add some stuff that makes creating the span easier to make it auto-closing based on scope.
FWIW the philosophy here is that observability is a part of the application rather than something separate. That's distinctly different from the APM philosophy, which is that a separate process "does the observability" and your app is "clean" from that. I think there's quite a benefit to manually instrumenting in your codebase intentionally rather than having an automated process do it for you. But I can understand not wanting to go through and do that.
That's an interesting distinction. Given some of the older, larger code bases, manually instrumenting seems like it would require a reasonably large effort. I think that's one of the huge benefits that some of the APM's bring.
I wonder if there is any reason OTEL can't have both.
OTel kind of has both today. Most languages support autoinstrumentation via either an agent, libraries, or both. So for example, if you've got a big Spring Boot app, it'll instrument requests/responses and DB calls for you. Some languages also have lightweight "sprinkle some spans on it" things you can put in code, like Java method annotations. But none have truly automatic "span for every method call" instrumentation.
FWIW the philosophy here is that observability is a part of the application rather than something separate. That's distinctly different from the APM philosophy, which is that a separate process "does the observability" and your app is "clean" from that. I think there's quite a benefit to manually instrumenting in your codebase intentionally rather than having an automated process do it for you. But I can understand not wanting to go through and do that.