Relabeling: Using a labelname in a regex (to use the labelname's value)

[thernstig]

In relabeling config, let's say I have saved a value in a label like __tmp_port. I then want to match some port in Kubernetes service discovery

  relabel_configs:
    - source_labels: [ __meta_kubernetes_pod_container_port_number ]
      action: keep
      regex: ${__tmp_port}

If the value of __meta_kubernetes_pod_container_port_number and __tmp_port are both 8080, it will keep this target. It could possibly be used in replacement as well.

Is this already possible somehow? Or would this be a good feature? It solves e.g. #3756