Revert "Revert to `id: :integer`"

This reverts commit 51340f9b47e8339cdabce11d0a17ac724278bff0.

db/migrate/20190513174947_enable_create_incident_issues_by_default.rb

+# frozen_string_literal: true
+
+class EnableCreateIncidentIssuesByDefault < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    change_default_for :create_issue, from: false, to: true
+    change_default_for :send_email, from: true, to: false
+  end
+
+  private
+
+  def change_default_for(column, from:, to:)
+    change_column_default :project_incident_management_settings,
+      column, from: from, to: to
+  end
+end

db/schema.rb

@@ -2503,9 +2503,9 @@ ActiveRecord::Schema.define(version: 20190620112608) do
     t.index ["project_id"], name: "index_project_import_data_on_project_id", using: :btree
   end
 
-  create_table "project_incident_management_settings", primary_key: "project_id", id: :integer, default: nil, force: :cascade do |t|
-    t.boolean "create_issue", default: false, null: false
-    t.boolean "send_email", default: true, null: false
+  create_table "project_incident_management_settings", primary_key: "project_id", id: :serial, force: :cascade do |t|
+    t.boolean "create_issue", default: true, null: false
+    t.boolean "send_email", default: false, null: false
     t.text "issue_template_key"
   end
 

doc/user/project/integrations/prometheus.md

@@ -160,7 +160,7 @@ receivers:
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/4925) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 11.11.
 
-Alerts can be used to trigger actions, like open an issue automatically. To configure the actions:
+Alerts can be used to trigger actions, like open an issue automatically (enabled by default since `12.1`). To configure the actions:
 
 1. Navigate to your project's **Settings > Operations > Incidents**.
 1. Enable the option to create issues.