Don't include tasks with no start date

Author: Abhishek Ray

airflow_prometheus_exporter/prometheus_exporter.py

@@ -120,6 +120,10 @@ def get_dag_duration_info():
                     == dag_start_dt_query.c.execution_date,
                 ),
             )
+            .filter(
+                TaskInstance.start_date.isnot(None),
+                TaskInstance.end_date.isnot(None),
+            )
             .all()
         )
 
@@ -479,7 +483,6 @@ def collect(self):
 if RBAC:
     from flask_appbuilder import BaseView as FABBaseView, expose as FABexpose
 
-
     class RBACMetrics(FABBaseView):
         route_base = "/admin/metrics/"