fix(flask): add regression test for active_requests gauge leak on error

Signed-off-by: alliasgher <alliasgher123@gmail.com>

Author: alliasgher

instrumentation/opentelemetry-instrumentation-flask/tests/test_programmatic.py

@@ -836,6 +836,36 @@ def test_duration_histogram_new_record_with_context_new_semconv(self):
                 span_arg.context.span_id, finished_span.context.span_id
             )
 
+    def test_active_requests_counter_decremented_on_error(self):
+        """Regression test for https://github.com/open-telemetry/opentelemetry-python-contrib/issues/4431.
+        http.server.active_requests must be decremented even when the view
+        raises an exception, so it does not permanently leak.
+        """
+        self.client.get("/hello/123")
+
+        resp = self.client.get("/hello/500")
+        self.assertEqual(500, resp.status_code)
+
+        metrics_list = self.memory_metrics_reader.get_metrics_data()
+        active_requests_value = None
+        for resource_metric in metrics_list.resource_metrics:
+            for scope_metric in resource_metric.scope_metrics:
+                for metric in scope_metric.metrics:
+                    if metric.name == "http.server.active_requests":
+                        for point in metric.data.data_points:
+                            if isinstance(point, NumberDataPoint):
+                                active_requests_value = point.value
+
+        self.assertIsNotNone(
+            active_requests_value,
+            "http.server.active_requests metric not found",
+        )
+        self.assertEqual(
+            0,
+            active_requests_value,
+            f"active_requests counter leaked: expected 0 but got {active_requests_value}",
+        )
+
 
 class TestProgrammaticHooks(InstrumentationTest, WsgiTestBase):
     def setUp(self):