tracing: Add __string_len() example

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>

samples/trace_events/trace-events-sample.h

@@ -303,6 +303,7 @@ TRACE_EVENT(foo_bar,
 		__bitmask(	cpus,	num_possible_cpus()	)
 		__cpumask(	cpum				)
 		__vstring(	vstr,	fmt,	va		)
+		__string_len(	lstr,	foo,	bar / 2 < strlen(foo) ? bar / 2 : strlen(foo) )
 	),
 
 	TP_fast_assign(
@@ -311,12 +312,13 @@ TRACE_EVENT(foo_bar,
 		memcpy(__get_dynamic_array(list), lst,
 		       __length_of(lst) * sizeof(int));
 		__assign_str(str, string);
+		__assign_str(lstr, foo);
 		__assign_vstr(vstr, fmt, va);
 		__assign_bitmask(cpus, cpumask_bits(mask), num_possible_cpus());
 		__assign_cpumask(cpum, cpumask_bits(mask));
 	),
 
-	TP_printk("foo %s %d %s %s %s %s (%s) (%s) %s", __entry->foo, __entry->bar,
+	TP_printk("foo %s %d %s %s %s %s %s (%s) (%s) %s", __entry->foo, __entry->bar,
 
 /*
  * Notice here the use of some helper functions. This includes:
@@ -360,7 +362,8 @@ TRACE_EVENT(foo_bar,
 		  __print_array(__get_dynamic_array(list),
 				__get_dynamic_array_len(list) / sizeof(int),
 				sizeof(int)),
-		  __get_str(str), __get_bitmask(cpus), __get_cpumask(cpum),
+		  __get_str(str), __get_str(lstr),
+		  __get_bitmask(cpus), __get_cpumask(cpum),
 		  __get_str(vstr))
 );