Supports external labels for prometheus queries

[killme2008]

What type of enhancement is this?

API improvement

What does the enhancement do?

Prometheus supports external labels:

# The labels to add to any time series or alerts when communicating with
  # external systems (federation, remote storage, Alertmanager). 
  # Environment variable references `${var}` or `$var` are replaced according 
  # to the values of the current environment variables. 
  # References to undefined variables are replaced by the empty string.
  # The `$` character can be escaped by using `$$`.
  external_labels:
    [ [<labelname>](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#labelname): [<labelvalue>](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#labelvalue) ... ]

For example, when you collect metrics from k8s cluster, the kube_node_infohas a column cluster which is a external label from prometheus.

And some Grafana dashboards need such labels for rendering.

So maybe we can choose to support the external labels as configuration per db:

[db.prometheus]
external_labels:
   cluster: test_cluster

Implementation challenges

No response