query: metadata endpoint /api/v1/label/<label_name>/values should return external labels

[FUSAKLA]

The metadata endpoint of Thanos Query /api/v1/label/<label_name>/values should return all values of the specified label name. (see Prometheus docs).

This works for all labels but not for the labels specified added by the external_labels of Thanos Store instances.

This is used for example by Grafana in label_values(<label_name>) which can be used to load lavel values to query variable see http://docs.grafana.org/features/datasources/prometheus/#query-variable.

[bwplotka]

Thanks, valid issue, PRs welcome (:

[bwplotka]

The actual work to be done, is to glue external labels from blocks in store Gateway.