Attach labels to prometheus scrape_config using rpk generate

[r-vasquez]

Following #5501 :

We want the option to add labels to the generated prometheus scrape_config via rpk, so:

$ rpk generate prometheus-config --seed-addr localhost:9092 --job-name redpanda-node --add-label cluster-id:cluster-a --public-metrics

will generate

- job_name: redpanda-node
  static_configs:
  - targets:
    - localhost:9644
  labels:
    - cluster-id: cluster-a
  metrics_path: /metrics
- job_name: redpanda-node-public
  static_configs:
  - targets:
    - localhost:9644
  labels:
    - cluster-id: cluster-a
  metrics_path: /public_metrics

[bpraseed]

Look like this fixes this ask #5548 ?

[r-vasquez]

Oh, it's duplicated. I'm closing this one.