Add new alert rules for throttling (#509)

- If OpenSearch is throttling, this is an alert that optimizations are
necessary like scaling the number of nodes or changing queries and
indexing patterns

src/alert_rules/prometheus/prometheus_alerts.yaml

@@ -128,3 +128,13 @@
     "for": "1m"
     "labels":
       "severity": "alert"
+
+  - "alert": "OpenSearchThrottling"
+    "annotations":
+      "message": "Cluster {{ $labels.cluster }} is throttling. Please optimize queries and indexing patterns or consider scale the application."
+      "summary": "OpenSearch Indexing Throttle"
+    "expr": |
+      sum by (cluster) (opensearch_indices_indexing_is_throttled_bool) > 0
+    "for": "5m"
+    "labels":
+      "severity": "warning"