Update index.md to use main

tutorial/content/labs/instrumentation/manual/metrics/index.md

@@ -149,7 +149,10 @@ from metric_utils import create_meter
 
 # global variables
 app = Flask(__name__)
-meter = create_meter("app.py", "0.1")
+
+if __name__ == "__main__":
+    # setup metrics
+    meter = create_meter("app.py", "0.1")
 ```
 
 Finally, open `app.py` and import `create_meter`.
@@ -783,4 +786,4 @@ As a result, the Prometheus server tries to bind to a port a second time, which
 To fix this, set the debug parameter to False.
 Start the app and open [localhost:8000](localhost:8000) in your browser.
 You should now see the metrics being exported in Prometheus text-based exposition format.
--->
+-->