Add a way to print debug counters without exiting

I am trying to study debug counters inside a Rails application.
Accessing debug counters by killing the process is hard because child
processes don't get the same TRAP as the parent, and Rails seems to
intercept calls to `exit`.  Adding this method lets me print the debug
counters when I want (at the end of requests for example)

debug_counter.c

@@ -43,6 +43,13 @@ rb_debug_counter_show_results(const char *msg)
     }
 }
 
+VALUE
+rb_debug_counter_show(void)
+{
+    rb_debug_counter_show_results("method call");
+    return Qnil;
+}
+
 VALUE
 rb_debug_counter_reset(void)
 {