debugging: add a perf trace example

[clementleger]

First: compile perf with trace support and then add a labs with perf trace

[Tropicao]

While the use case is legitimate, I fear that adding a third tracing solution (on top of raw tracefs manipulations and trace-cmd) would bring more confusion than help: after introducing perf, ftrace and trace-cmd, trainees are often confused about each tool initial purpose.

If the main interest in perf-trace is about the fact that traces are emitted in the console directly (instead of generating a recording file), we should rather mention trace-cmd stream, but I feel like it can be kept as a non-written addition for additional questions.

Closing this for now, but anyone is welcome to prove me wrong and if so, to reopen it.