Add Cronitor decorator collate-letter-pdfs-for-day

This celery task was not decorated with the cronitor decorator so never
checked in with cronitor.

Adding the decorator will ensure this task is monitored.

The requisite cronitor key is in the credentials repository already.

Signed-off-by: Toby Lorne <toby.lornewelch-richards@digital.cabinet-office.gov.uk>

app/celery/letters_pdf_tasks.py

@@ -47,6 +47,7 @@ from app.models import (
     NOTIFICATION_VALIDATION_FAILED,
     NOTIFICATION_VIRUS_SCAN_FAILED,
 )
+from app.cronitor import cronitor
 
 
 @notify_celery.task(bind=True, name="create-letters-pdf", max_retries=15, default_retry_delay=300)
@@ -110,6 +111,7 @@ def get_letters_pdf(template, contact_block, filename, values):
     return resp.content, billable_units
 
 
+@cronitor("collate-letter-pdfs-for-day")
 @notify_celery.task(name='collate-letter-pdfs-for-day')
 def collate_letter_pdfs_for_day(date=None):
     if not date: