Make `identifier:` unique

db/migrate/20180831164908_add_identifier_to_prometheus_metric.rb

@@ -6,6 +6,7 @@ class AddIdentifierToPrometheusMetric < ActiveRecord::Migration
   DOWNTIME = false
 
   def change
-    add_column :prometheus_metrics, :identifier, :string, unique: true
+    add_column :prometheus_metrics, :identifier, :string
+    add_index :prometheus_metrics, :identifier, unique: true
   end
 end

db/schema.rb

@@ -1712,6 +1712,7 @@ ActiveRecord::Schema.define(version: 20180831164909) do
 
   add_index "prometheus_metrics", ["common"], name: "index_prometheus_metrics_on_common", using: :btree
   add_index "prometheus_metrics", ["group"], name: "index_prometheus_metrics_on_group", using: :btree
+  add_index "prometheus_metrics", ["identifier"], name: "index_prometheus_metrics_on_identifier", unique: true, using: :btree
   add_index "prometheus_metrics", ["project_id"], name: "index_prometheus_metrics_on_project_id", using: :btree
 
   create_table "protected_branch_merge_access_levels", force: :cascade do |t|