[ruler] Support external labels

Is your feature request related to a problem? Please describe.

It's not possible to set extra labels on series at the ruler, like it is with Prometheus with external_labels. So you can't set e.g. 'cluster' or 'environment' etc.

Describe the solution you'd like

Something like what Prometheus provides:

global:
  # The labels to add to any time series or alerts when communicating with
  # external systems (federation, remote storage, Alertmanager).
  external_labels:
    [ [<labelname>](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#labelname): [<labelvalue>](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#labelvalue) ... ]