global.external_labels - When should I expect to see them in my metrics?

[Aaron Layfield]

In my case I am trying to use external_labels for applying a standard cluster name label for the kubernetes_cluster.

I am expecting that when an alert triggers via the alertmanager, this label should be present in the output.

However, it seems like the label is not present or I have done something wrong.

This is how the Prometheus config looks:

```

  prometheus.yml: |
    global:
      external_labels:
        kubernetes_cluster_name: xyz

```

But the alert comes out of alertmanager without that label:

Under what conditions should we expect to see the external_labels?

[Aaron Layfield]

Bumping this as it would be great to understand the conditions where `external_labels` are expected to be seen.