In order to do efficient routing we should enable the Admins to set cluster level labels, that will apply to all firing alerts.
For example: env=prod, geo=emea , cluster_name=clusterA .

This is needed to enable efficient routing of alerts.
The way to configure this is by setting the `externalLabels` that is in the Prometheus Config.
For the UI we need a way to add/update/delete labels with API calls.

This is critical for multi-cluster alerts management and routing.