Add alert for Fastly cost

If the cost exceeds $0, it indicates that we have run out of credit and/or are doing something wrong, in either case we want to be alerted.

roles/prometheus/files/node.rules.yml

@@ -504,7 +504,7 @@ groups:
   - name: fastly
     interval: 60m
     rules:
-      - alert: RebuilderdQueueNotEmpty
+      - alert: FastlyCostNotZero
         expr: max_over_time(fastly_mtd_estimate_dollars_total[2h]) > 0
         labels:
           severity: warning