Add Monitors in Datadog for Pager Duty

**Acceptance Criteria**
Add Monitors for the following (this list is WIP):

- [x] High number of 500s
- [ ] High Number of instances (indicates Autoscaling is working, but consuming too many resources)
- [x] High Latency on Page Load (indicates overall site performance degradation)
- [ ] High number of jobs enqueued in Redis (indicates celery workers aren't keeping up with demand)
- [x] Synthetic pageload tests failing (canary uptime test)

- [ ] Ensure Read Replica DBs are also monitored
- [x] High Mem on DBs
- [x] High CPU on DBs (DB load is a known bottleneck, may need to find and kill long running queries)
- [x] High Latency on DB Querries (indicates inefficient queries, or high db load)
- [x] High CPU on cluster (indicates Autoscaling is lagging behind demand)

**XD Links:**

**Tech Details:**

**Open Questions:**

**Notes/Assumptions:**