Tracer, debugger: refactoring to reduce boilerplate

[ccoVeille]

I thi k it's important to avoid the boilerplate we have everywhere. I mean code that is copy pasted and is pain to fix like #1214

I think about a short lived struct. Based in what have been done with #1235.

In other frameworks it would be a middleware that is added on demand, without interference with the code.

I'm unsure how easy it would be to consider a middleware

Originally posted by @ccoVeille in #1235 (comment)

The struct could be created via a constructor, that will return a closer func.

I'm unsure yet if it should create the span or not.