ref(metrics-layer): Test the span metric charts too (#54045)

- This adds the metrics-layer feature flag to the span metrics
event-stats tests too

tests/snuba/api/endpoints/test_organization_events_stats_span_metrics.py

@@ -161,3 +161,11 @@ class OrganizationEventsStatsSpansMetricsEndpointTest(MetricsEnhancedPerformance
             rows = data[0:6]
             for test in zip(event_counts, rows):
                 assert test[1][1][0]["count"] == test[0] / 60.0
+
+
+class OrganizationEventsStatsSpansMetricsEndpointTestWithMetricLayer(
+    OrganizationEventsStatsSpansMetricsEndpointTest
+):
+    def setUp(self):
+        super().setUp()
+        self.features["organizations:use-metrics-layer"] = True