Move saving of processing time into separate task

We current do this as part of send-daily-performance-platform-stats but
now this moves it into its own separate task. This is for two reasons
- we will shortly get rid of the send-daily-performance-platform-stats
  task as we no longer will need to send anything to performance
  platform
- even if we did decide to keep the task
  send-daily-performance-platform-stats and remove the specific bits
  that relate to the performance platform, it's probably nicer to
  rewrite the new task from scratch to make sure it's all clear and easy
  to understand

app/celery/nightly_tasks.py

@@ -14,12 +14,14 @@ from app.celery.service_callback_tasks import (
 )
 from app.config import QueueNames
 from app.cronitor import cronitor
+from app.dao.fact_processing_time_dao import insert_update_processing_time
 from app.dao.inbound_sms_dao import delete_inbound_sms_older_than_retention
 from app.dao.jobs_dao import (
     dao_archive_job,
     dao_get_jobs_older_than_data_retention,
 )
 from app.dao.notifications_dao import (
+    dao_get_total_notifications_sent_per_day_for_performance_platform,
     dao_timeout_notifications,
     delete_notifications_older_than_retention_by_type,
 )
@@ -33,6 +35,7 @@ from app.models import (
     LETTER_TYPE,
     NOTIFICATION_SENDING,
     SMS_TYPE,
+    FactProcessingTime,
     Notification,
 )
 from app.performance_platform import processing_time, total_sent_notifications
@@ -318,3 +321,26 @@ def letter_raise_alert_if_no_ack_file_for_zip():
         current_app.logger.info(
             "letter ack contains zip that is not for today: {}".format(ack_file_set - zip_file_set)
         )
+
+
+@notify_celery.task(name='save-daily-notification-processing-time')
+@cronitor("save-daily-notification-processing-time")
+@statsd(namespace="tasks")
+def save_daily_notification_processing_time(bst_date=None):
+    # bst_date is a string in the format of "YYYY-MM-DD"
+    if bst_date is None:
+        # if a date is not provided, we run against yesterdays data
+        bst_date = (datetime.utcnow() - timedelta(days=1)).date()
+    else:
+        bst_date = datetime.strptime(bst_date, "%Y-%m-%d").date()
+
+    start_time = get_london_midnight_in_utc(bst_date)
+    end_time = get_london_midnight_in_utc(bst_date + timedelta(days=1))
+    result = dao_get_total_notifications_sent_per_day_for_performance_platform(start_time, end_time)
+    insert_update_processing_time(
+        FactProcessingTime(
+            bst_date=bst_date,
+            messages_total=result.messages_total,
+            messages_within_10_secs=result.messages_within_10_secs
+        )
+    )