Test out external_labels functionality for Prometheus CR

[rexagod]

I think that using the external_labels setting in prometheus configuration would fit very well with our requirements, (and less code is always better.) In any case your proposal is ok for me.

https://prometheus.io/docs/prometheus/latest/configuration/configuration/

# The labels to add to any time series or alerts when communicating with
# external systems (federation, remote storage, Alertmanager).
external_labels:
    [ [<labelname>]: [<labelvalue>]

Originally posted by @jmolmo in #69 (comment)