DEV-517: added alerting for DO API usage

templates/prometheus/config/prometheus/alert.rules.j2

@@ -332,3 +332,11 @@ groups:
       summary: "one or more offsite backup metric unavailable for Instance <{{ '{{' }} $labels.instance {{ '}}' }}>"
       description: "offsite metrics unavailable."
 
+  - alert: DO_API_REQUEST_usage
+    expr: (digitalocean_api_ratelimit_remaining * 100)/digitalocean_api_ratelimit_limit < 50
+    for: 10m
+    labels:
+      severity: critical
+    annotations:
+      summary: "Too many request against digitalocean API. plz check."
+      description: "Alert for request against digitalocean API."