Prometheus Adapter emtpy custom metrics

Question

I've installed prometehus adapter with the following command:

helm install prometheus-adapter prometheus-community/prometheus-adapter --namespace monitoring
-f adapter_custom_values.yaml

However when checking with kubectl get --raw /apis/custom.metrics.k8s.io/v1beta1, I dont see any metrics being registered. Here is a preview of the adapter_custom_values.yaml file:

prometheus:

     # Run `kubectl get svc -n namespace` to get the url of Prometheus
    
      url: http://prometheus-kube-prometheus-prometheus.monitoring.svc

      port: 9090
    
    logLevel: 10
    
    replicas: 1
    
    rules:
    
      # Whether to enable the default rules
    
      default: false
    
      # Add custom metrics API here
    
      custom:
    
      # plex_queue_size API
    
      - seriesQuery: 'plex_queue_size'
    
        resources:
    
          overrides:
    
            namespace: {resource: "namespace"}
    
            pod: {resource: "pod"}
    
        name:
    
          matches: ^(.*)
    
          as: "plex_queue_size"
    
        metricsQuery: <<.Series>>{<<.LabelMatchers>>}

But when checking the custom metrics api with the command kubectl get --raw /apis/custom.metrics.k8s.io/v1beta1 , I am getting empty response:

{
  "kind": "APIResourceList",
  "apiVersion": "v1",
  "groupVersion": "custom.metrics.k8s.io/v1beta1",
  "resources": []
}

From the adapter logs i can see that the request to prometheus is succesfull but the data inside of the Responsde Body is empty:

GET http://prometheus-kube-prometheus-prometheus.monitoring.svc:9090/api/v1/series?match%5B%5D=plex_queue_size&start=1640091045.342 200 OK
Response Body: {"status":"success","data":[]}
Set available metric list from Prometheus to: [[]] .

Kube-prometheus-stack is also installed with helm.

My question is why the custom metrics are't being registered in the Custom Metrics API ? Any help would be appreciated.