Capture code attributes in Ratpack instrumentation

[trask]

Part of #7345

[SylvainJuge]

I remember we had a similar discussion a while ago (in the context of #7345), but do you think we could capture the user/application code that implements the handler instead of the ratpack implementation class ?

While it is not written explicitly in semantic conventions, giving priority to classes/methods of the application is expected to provide "more value" in the sense that the consumer of this data has more leverage on the application implementation than on the underlying framework.

We could also implement a few alternatives:

• capture two spans: one for the framework, one for the "user/application code", the downside is that they would always have similar duration, but it could allow to compare usages of the same framework feature across applications for example
• extend semantic conventions to provide a clear split between "technical" or "framework" code attributes and the "application".

In think that having a common strategy to capture it would probably be beneficial, especially if we want to move forward the work on #7345.

[laurit]

I totally agree that adding TracingHandler here isn't useful and using the actual user class that handles the request would be much better. The only problem is that getting that class isn't easy.