title: Prometheus multiple source label in relabel config
body: <p>For some reason, my Pod in k8s could possibly labeled with <code>syscode</code> or <code>cib_syscode</code>, how can I relabel both of them to <code>cib_syscode</code>? That is to say if pod labeled with one of them, choose its label value as the result.
  I have tried as follow, but it doesn't work because the target_label is duplicate.</p>
  <pre class="lang-yaml prettyprint-override"><code>    - job_name: ncr
        honor_labels: true
        scrape_interval: 30s
        scrape_timeout: 30s
        metrics_path: /metrics
        scheme: http
        kubernetes_sd_configs:
        - role: pod
        relabel_configs:
        - source_labels: [__meta_kubernetes_pod_label_syscode]
          separator: ;
          regex: (.*)
          target_label: cib_syscode
          replacement: ${1}
          action: replace
        - source_labels: [__meta_kubernetes_pod_label_cib_syscode]
          separator: ;
          regex: (.*)
          target_label: cib_syscode
          replacement: ${1}
          action: replace
  </code></pre>
  <p>Looking forward to your reply, thank you!</p>