Update RabbitMQ scaler docs for adding custom metric name support

Signed-off-by: Ratnadeep Debnath <rtnpro@gmail.com>

content/docs/2.4/scalers/rabbitmq-queue.md

@@ -39,6 +39,7 @@ triggers:
 - `queueLength`: DEPRECATED! Use `mode: QueueLength` and `value: ##` instead. Target value for queue length passed to the scaler. Example: if one pod can handle 10 messages, set the queue length target to 10. If the actual number of messages in the queue is 30, the scaler scales to 3 pods. Default is 20 unless `publishRate` is specified, in which case `queueLength` is disabled for this trigger.
 - `useRegex`: In case of `http` protocol, this parameter allows to use regex (in `queueName` parameter) to select queue instead of full name, the valid values are: `"true"` and `"false"`.  Optional.
 - `operation`: Operation that will be applied to compute the number of messages in case of `useRegex` enabled. Either `sum` (default),`max`, or `avg`. Optional.
+- `metricName` - an optional name to assign to the metric. If not set KEDA will generate a name based on the queue name. If using more than one trigger it is required that all metricNames be unique.
 
 Some parameters could be provided using environmental variables, instead of setting them directly in metadata. Here is a list of parameters you can use to retrieve values from environment variables:
 
@@ -101,6 +102,7 @@ spec:
       queueName: testqueue
       mode: QueueLength
       value: "20"
+      metricName: custom-testqueue #optional. Generated value would be `rabbitmq-custom-testqueue`
     authenticationRef:
       name: keda-trigger-auth-rabbitmq-conn
 ```