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

tests/app/celery/test_nightly_tasks.py

@@ -19,6 +19,7 @@ from app.celery.nightly_tasks import (
     remove_letter_csv_files,
     remove_sms_email_csv_files,
     s3,
+    save_daily_notification_processing_time,
     send_daily_performance_platform_stats,
     send_total_sent_notifications_to_performance_platform,
     timeout_notifications,
@@ -31,7 +32,7 @@ from app.clients.performance_platform.performance_platform_client import (
 )
 from app.config import QueueNames
 from app.exceptions import NotificationTechnicalFailureException
-from app.models import EMAIL_TYPE, LETTER_TYPE, SMS_TYPE
+from app.models import EMAIL_TYPE, LETTER_TYPE, SMS_TYPE, FactProcessingTime
 from tests.app.db import (
     create_ft_notification_status,
     create_job,
@@ -439,3 +440,77 @@ def test_letter_not_raise_alert_if_no_files_do_not_cause_error(mocker, notify_db
     letter_raise_alert_if_no_ack_file_for_zip()
 
     assert mock_file_list.call_count == 2
+
+
+@freeze_time('2021-01-18T02:00')
+@pytest.mark.parametrize('date_provided', [None, '2021-1-17'])
+def test_save_daily_notification_processing_time(mocker, sample_template, date_provided):
+    # notification created too early to be counted
+    create_notification(
+        sample_template,
+        created_at=datetime(2021, 1, 16, 23, 59),
+        sent_at=datetime(2021, 1, 16, 23, 59) + timedelta(seconds=5)
+    )
+    # notification counted and sent within 10 seconds
+    create_notification(
+        sample_template,
+        created_at=datetime(2021, 1, 17, 00, 00),
+        sent_at=datetime(2021, 1, 17, 00, 00) + timedelta(seconds=5)
+    )
+    # notification counted but not sent within 10 seconds
+    create_notification(
+        sample_template,
+        created_at=datetime(2021, 1, 17, 23, 59),
+        sent_at=datetime(2021, 1, 17, 23, 59) + timedelta(seconds=15)
+    )
+    # notification created too late to be counted
+    create_notification(
+        sample_template,
+        created_at=datetime(2021, 1, 18, 00, 00),
+        sent_at=datetime(2021, 1, 18, 00, 00) + timedelta(seconds=5)
+    )
+
+    save_daily_notification_processing_time(date_provided)
+
+    persisted_to_db = FactProcessingTime.query.all()
+    assert len(persisted_to_db) == 1
+    assert persisted_to_db[0].bst_date == date(2021, 1, 17)
+    assert persisted_to_db[0].messages_total == 2
+    assert persisted_to_db[0].messages_within_10_secs == 1
+
+
+@freeze_time('2021-04-18T02:00')
+@pytest.mark.parametrize('date_provided', [None, '2021-4-17'])
+def test_save_daily_notification_processing_time_when_in_bst(mocker, sample_template, date_provided):
+    # notification created too early to be counted
+    create_notification(
+        sample_template,
+        created_at=datetime(2021, 4, 16, 22, 59),
+        sent_at=datetime(2021, 4, 16, 22, 59) + timedelta(seconds=15)
+    )
+    # notification counted and sent within 10 seconds
+    create_notification(
+        sample_template,
+        created_at=datetime(2021, 4, 16, 23, 00),
+        sent_at=datetime(2021, 4, 16, 23, 00) + timedelta(seconds=5)
+    )
+    # notification counted and sent within 10 seconds
+    create_notification(
+        sample_template,
+        created_at=datetime(2021, 4, 17, 22, 59),
+        sent_at=datetime(2021, 4, 17, 22, 59) + timedelta(seconds=5)
+    )
+    # notification created too late to be counted
+    create_notification(
+        sample_template,
+        created_at=datetime(2021, 4, 17, 23, 00),
+        sent_at=datetime(2021, 4, 17, 23, 00) + timedelta(seconds=15)
+    )
+
+    save_daily_notification_processing_time(date_provided)
+
+    persisted_to_db = FactProcessingTime.query.all()
+    assert len(persisted_to_db) == 1
+    assert persisted_to_db[0].bst_date == date(2021, 4, 17)
+    assert persisted_to_db[0].messages_total == 2
+    assert persisted_to_db[0].messages_within_10_secs == 2