Consider creating an observability section containing logging, metrics and tracing

[philwebb]

We currently have tracing and http tracing, I wonder if we should merge them?

[philwebb]

We're going to create an Observability section and move logging, metrics and tracing under it. We're going to rename the /httptraces actuator endpoint to /httpexchanges

[philwebb]

With #32885 fixed this doesn't seem as urgent. I think we should leave the docs as they are for 3.0