Add monitoring and logging

docs/monitoring.md

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+# Infrastructure Metric monitoring
+
+We use self-hosted Prometheus and Grafana for monitoring infrastructure metrics. These are run in a dedicated Gov PaaS space called "monitoring" and are deployed as Docker images using Github action pipelines. The repository for these and more information is here: [dluhc-data-collection-monitoring](https://github.com/communitiesuk/dluhc-data-collection-monitoring).
+
+# Application & Performance monitoring & alerting
+
+For application error and performance monitoring we use managed [Sentry](https://sentry.io/organizations/dluhc-core). You will need to be added to the DLUHC account to access this. It triggers slack notifications to the #team-data-collection-alerts channel for all application errors in staging and production and for any controller endpoints that have a P95 transaction duration > 250ms over a 24 hour period.
+
+# Logs
+
+For log persistence we use a managed ELK (Elasticsearch, Logstash, Kibana) stack provided by [Logit](https://logit.io/). You will need to be added to the DLUHC account to access this. Longs are retained for 14 days with a daily limit of 2GB.
+
+Logs are also available from Gov PaaS directly via cli `cf logs <gov-paas-space-name> --recent`.