Reduce test_backend_transaction_age flakiness

Add the timestamp to the application name to reduce the risk of leftover
query and connection being present when metrics are collected.

Author: bonnefoa

postgres/tests/test_pg_integration.py

@@ -485,7 +485,7 @@ def test_backend_transaction_age(aggregator, integration_check, pg_instance):
 
     check.run()
 
-    app = 'test_backend_transaction_age'
+    app = f'test_backend_transaction_age_{time.time()}'
     conn1 = _get_conn(pg_instance, application_name=app)
     cur = conn1.cursor()