Show alerts settings only for manual configuration

We should hide alerts settings from GitLab-managed Prometheus
instances and show them only in manual mode.

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/9258#note_132612251

ee/app/views/projects/services/prometheus/_external_alerts.html.haml

 - return unless can?(current_user, :read_prometheus_alerts, @project)
+- return unless @service.manual_configuration?
 
 - notify_url = notify_project_prometheus_alerts_url(@project, format: :json)
 - authorization_key = @project.alerting_setting.try(:token)

ee/changelogs/unreleased/pl-fix-show-external-alerts.yml

+---
+title: Show alerts settings only for manual configuration
+merge_request: 9538
+author:
+type: fixed

ee/spec/features/projects/services/prometheus_external_alerts_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Prometheus external alerts', :js do
+  let(:project) { create(:project) }
+  let(:user) { create(:user) }
+
+  let(:alerts_section_selector) { '.js-prometheus-alerts' }
+  let(:alerts_section) { page.find(alerts_section_selector) }
+
+  before do
+    stub_licensed_features(prometheus_alerts: true)
+
+    sign_in(user)
+    project.add_maintainer(user)
+
+    visit_edit_service
+  end
+
+  context 'with manual configuration' do
+    before do
+      check('Active')
+      fill_in('API URL', with: 'http://prometheus.example.com')
+      click_button('Save changes')
+
+      visit_edit_service
+    end
+
+    it 'shows the Alerts section' do
+      wait_for_requests
+
+      expect(alerts_section).to have_content('Alerts')
+      expect(alerts_section).to have_content('Receive alerts from manually configured Prometheus servers.')
+      expect(alerts_section).to have_content('URL')
+      expect(alerts_section).to have_content('Authorization key')
+    end
+  end
+
+  context 'with no configuration' do
+    it 'does not show the Alerts section' do
+      wait_for_requests
+
+      expect(page).not_to have_css(alerts_section_selector)
+    end
+  end
+
+  private
+
+  def visit_edit_service
+    visit(project_settings_integrations_path(project))
+    click_link('Prometheus')
+  end
+end