Correct timer units to seconds from milliseconds. (#27360)

``dagrun.duration.success.<dag_id>``
``dagrun.schedule_delay.<dag_id>``
``dagrun.<dag_id>.first_task_scheduling_delay`` are reported to statsd in seconds, not milliseconds. Other timers in this list might be suspect, but I haven't checked them yet.

(cherry picked from commit 25d7ca9)

docs/apache-airflow/logging-monitoring/metrics.rst

@@ -155,12 +155,12 @@ Name                                                Description
 ``dagrun.dependency-check.<dag_id>``                Milliseconds taken to check DAG dependencies
 ``dag.<dag_id>.<task_id>.duration``                 Milliseconds taken to finish a task
 ``dag_processing.last_duration.<dag_file>``         Milliseconds taken to load the given DAG file
-``dagrun.duration.success.<dag_id>``                Milliseconds taken for a DagRun to reach success state
+``dagrun.duration.success.<dag_id>``                Seconds taken for a DagRun to reach success state
 ``dagrun.duration.failed.<dag_id>``                 Milliseconds taken for a DagRun to reach failed state
-``dagrun.schedule_delay.<dag_id>``                  Milliseconds of delay between the scheduled DagRun
+``dagrun.schedule_delay.<dag_id>``                  Seconds of delay between the scheduled DagRun
                                                     start date and the actual DagRun start date
 ``scheduler.critical_section_duration``             Milliseconds spent in the critical section of scheduler loop --
                                                     only a single scheduler can enter this loop at a time
-``dagrun.<dag_id>.first_task_scheduling_delay``     Milliseconds elapsed between first task start_date and dagrun expected start
+``dagrun.<dag_id>.first_task_scheduling_delay``     Seconds elapsed between first task start_date and dagrun expected start
 ``collect_db_dags``                                 Milliseconds taken for fetching all Serialized Dags from DB
 =================================================== ========================================================================