Add on_delete :cascade and fix testcase description

db/schema.rb

@@ -2587,7 +2587,7 @@ ActiveRecord::Schema.define(version: 20180115201419) do
   add_foreign_key "project_import_data", "projects", name: "fk_ffb9ee3a10", on_delete: :cascade
   add_foreign_key "project_mirror_data", "projects", name: "fk_d1aad367d7", on_delete: :cascade
   add_foreign_key "project_statistics", "projects", on_delete: :cascade
-  add_foreign_key "prometheus_metrics", "projects"
+  add_foreign_key "prometheus_metrics", "projects", on_delete: :cascade
   add_foreign_key "protected_branch_merge_access_levels", "namespaces", column: "group_id", name: "fk_98f3d044fe", on_delete: :cascade
   add_foreign_key "protected_branch_merge_access_levels", "protected_branches", name: "fk_8a3072ccb3", on_delete: :cascade
   add_foreign_key "protected_branch_merge_access_levels", "users"

spec/models/prometheus_metric_spec.rb

@@ -8,7 +8,7 @@ describe PrometheusMetric do
   it { is_expected.to validate_presence_of(:query) }
   it { is_expected.to validate_presence_of(:group) }
 
-  describe '#group_text' do
+  describe '#group_title' do
     shared_examples 'group_title' do |group, text|
       subject { build(:prometheus_metric, group: group).group_title }