Add C# examples to CPU optimization

Co-authored-by: A Thousand Ships <96648715+AThousandShips@users.noreply.github.com>

tutorials/performance/cpu_optimization.rst

@@ -92,7 +92,8 @@ using a profiler, is to manually time the function or area under test.
 The specifics vary depending on the language, but in GDScript, you would do
 the following:
 
-::
+.. tabs::
+ .. code-tab:: gdscript GDScript
 
     var time_start = Time.get_ticks_usec()
 
@@ -102,6 +103,16 @@ the following:
     var time_end = Time.get_ticks_usec()
     print("update_enemies() took %d microseconds" % time_end - time_start)
 
+ .. code-tab:: csharp
+
+    var timeStart = Time.GetTicksUsec();
+
+    // Your function you want to time.
+    UpdateEnemies();
+
+    var timeEnd = Time.GetTicksUsec();
+    GD.Print($"UpdateEnemies() took {timeEnd - timeStart} microseconds");
+
 When manually timing functions, it is usually a good idea to run the function
 many times (1,000 or more times), instead of just once (unless it is a very slow
 function). The reason for doing this is that timers often have limited accuracy.