Add RabbitMQ file descriptors alerts

- warning - 70%
- critical - 95%

Change-Id: I697ac0f7faa587be00fcb1bec64206a0e4cd608b
Related-bug: PROD-33352
(cherry picked from commit fbc2fe83476de16ca6a569deb042e1cc584d1902)
diff --git a/rabbitmq/meta/prometheus.yml b/rabbitmq/meta/prometheus.yml
index 3dcfcf6..a8cc59b 100644
--- a/rabbitmq/meta/prometheus.yml
+++ b/rabbitmq/meta/prometheus.yml
@@ -7,6 +7,26 @@
 {%- raw %}
 server:
   alert:
+    RabbitmqFdUsageWarning:
+      if: >-
+        rabbitmq_node_fd_used / rabbitmq_node_fd_total * 100 >= 70
+      for: 1m
+      labels:
+        severity: warning
+        service: rabbitmq
+      annotations:
+        summary: "RabbitMQ service uses 70% of file descriptors"
+        description: "The RabbitMQ service uses {{ $value }}% of all available file descriptors on the {{ $labels.host }} node."
+    RabbitmqFdUsageCritical:
+      if: >-
+        rabbitmq_node_fd_used / rabbitmq_node_fd_total * 100 >= 95
+      for: 1m
+      labels:
+        severity: critical
+        service: rabbitmq
+      annotations:
+        summary: "RabbitMQ service uses 95% of file descriptors"
+        description: "The RabbitMQ service uses {{ $value }}% of all available file descriptors on the {{ $labels.host }} node."
     RabbitmqServiceDown:
       if: >-
         rabbitmq_up == 0