Added labels_hook_attrs method

Based on review comment fetching labels hook_attrs is now
wrapped in an issue's model method.

app/models/issue.rb

@@ -254,6 +254,10 @@ class Issue < ApplicationRecord
     merge_requests_closing_issues.count
   end
+  def labels_hook_attrs
+    labels.map(&:hook_attrs)
+  end
+
   private
   def ensure_metrics

lib/gitlab/data_builder/note.rb

@@ -44,7 +44,7 @@ module Gitlab
           data[:commit] = build_data_for_commit(project, user, note)
         elsif note.for_issue?
           data[:issue] = note.noteable.hook_attrs
-          data[:issue][:labels] = note.noteable.labels.map(&:hook_attrs)
+          data[:issue][:labels] = note.noteable.labels_hook_attrs
         elsif note.for_merge_request?
           data[:merge_request] = note.noteable.hook_attrs
         elsif note.for_snippet?

lib/gitlab/hook_data/issue_builder.rb

@@ -45,7 +45,7 @@ module Gitlab
             human_time_estimate: issue.human_time_estimate,
             assignee_ids: issue.assignee_ids,
             assignee_id: issue.assignee_ids.first, # This key is deprecated
-            labels: issue.labels.map(&:hook_attrs)
+            labels: issue.labels_hook_attrs
         }
         issue.attributes.with_indifferent_access.slice(*self.class.safe_hook_attributes)

spec/models/issue_spec.rb

@@ -862,4 +862,13 @@ describe Issue do
       end
     end
   end
+
+  describe "#labels_hook_attrs" do
+    let(:label) { create(:label) }
+    let(:issue) { create(:labeled_issue, labels: [label]) }
+
+    it "returns a list of label hook attributes" do
+      expect(issue.labels_hook_attrs).to eq([label.hook_attrs])
+    end
+  end
 end