Batch input data and send parallel http calls

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Hello,
I am trying to do the following:
Take input on tcp and send it to a http endpoint. I need to send batches of events to the http endpoint. I am trying to use the Aggregate filter to batch events and then sending to http endpoint as a json array. Here is what the config looks like:

input {
	tcp {
		port => 8081
	}
}

filter {

  ruby {
    code => 'event.set("randomNumber", rand(10))'
  }
  aggregate {
    task_id => "%{randomNumber}"
    timeout => 5
    push_map_as_event_on_timeout => true
    code => "
        map['messages'] ||= []
        map['messages'] << event.get('message')
        event.cancel()
    "
  }

  json_encode {
    source => "messages"
    target => "messages"
  }

}

output {

http {
        http_method => "post"
        headers => ["Content-Type", "application/json", "Content-Encoding", "gzip"]
        url => "http://localhost:8086/logs/json"
        format => "message"
        message => "%{messages}"
        content_type => "application/json"
        http_compression => "true"
    }
}

pipeline workers is set to 20

My expectation is:
The aggregate filter will put the incoming data in 10 random buckets and send the bucket after 5s timeout to the output. I am expecting to see upto 10 parallel http calls.
However, I only see one call and if that call is slow then no other http call happens.