tracing: Add __string_len() example

tracing: Add __string_len() example

[ Upstream commit dd6ae6d90a84d4bec49887c7aa2b22aa1c8b2897 ]

There's no example code that uses __string_len(), and since the sample
code is used for testing the event logic, add a use case.

Link: https://lore.kernel.org/linux-trace-kernel/20240223152827.5f9f78e2@gandalf.local.home
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Stable-dep-of: ea8d7647f9dd ("tracing: Verify event formats that have "%*p.."")
Signed-off-by: Sasha Levin <sashal@kernel.org>