Add ADR describing decision to add Sentry monitoring

docs/adr/0003-use-sentry-cron-monitoring.md

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+# 2. Use Sentry Cron monitoring
+
+Date: 2024-06-26
+
+## Status
+
+Accepted
+
+## Context
+
+The metrics tasks run as a dokku-controlled cron job.
+
+There have been instances when this cron job has failed silently due to software or configuration errors.
+We would like to know if there have been errors or if the cron job has not run on schedule.
+
+The `dokku logs metrics` command did not reveal any trace of these silent errors.
+
+Sentry is in use for other monitoring of systems within the Bennett Institute.
+Sentry [offers](https://docs.sentry.io/platforms/python/crons/) the ability to monitor scheduled cron jobs.
+Sentry Cron monitoring allows [check-ins](https://docs.sentry.io/platforms/python/crons/#manual-check-ins)
+to be sent at various points in the job's execution with status codes, but not full error messages.
+
+Sentry also offers [error capture](https://docs.sentry.io/product/sentry-basics/integrate-backend/capturing-errors/)
+and reporting functionality, which is in use in other Bennett Institute projects.
+
+## Decision
+
+We will use Sentry Cron monitoring to monitor the execution of the metrics cron job on dokku3.
+
+We will not implement Sentry error capture at this point in time.
+
+## Consequences
+
+We will know if the metrics cron job is executing according to schedule, and if there are any errors or not.
+
+We will not know via Sentry what these errors are (if any).
+Further manual investigation will be required.
+If this becomes a problem in future, we can implement Sentry error capture.