Add a stub prometheus[external_labels] configuration

files/gitlab-config-template/gitlab.rb.template

@@ -1639,6 +1639,7 @@ gitlab_rails['object_store']['objects']['terraform_state']['bucket'] = nil
 # prometheus['rules_files'] = ['/var/opt/gitlab/prometheus/rules/*.rules']
 # prometheus['scrape_interval'] = 15
 # prometheus['scrape_timeout'] = 15
+# prometheus['external_labels'] = { }
 # prometheus['env_directory'] = '/opt/gitlab/etc/prometheus/env'
 # prometheus['env'] = {
 #   'SSL_CERT_DIR' => "/opt/gitlab/embedded/ssl/certs/"