Passing kubernetes cluster name to Promtail

I want to pass kubernetes cluster name thru environment variables to Promtail agent as an extra label. Any suggestions?

This is how my config looks like

          - source_labels: 
            - __env_CLUSTER_NAME
            target_label: cluster_name 

but I still can not see the cluster_name label.

I set the cluster name as an ENV variable and use it in my Promtail config to set external_labels on my client.

  - basic_auth:
      password: ${LOKI_BASIC_AUTH_PW}
      username: ${LOKI_BASIC_AUTH_USER}
      cluster: ${CLUSTER}
    url: ${LOKI_URL}