Move parsing to alert

ee/app/presenters/projects/prometheus/alert_presenter.rb

@@ -42,10 +42,6 @@ module Projects
         MARKDOWN
       end
 
-      def alert_markdown
-        annotations.find { |annotation| annotation.label == 'gitlab_incident_markdown' }&.value
-      end
-
       private
 
       def alert_title

ee/lib/gitlab/alerting/alert.rb

@@ -49,6 +49,12 @@ module Gitlab
         end
       end
 
+      def alert_markdown
+        strong_memoize(:alert_markdown) do
+          parse_alert_markdown_from_payload
+        end
+      end
+
       def valid?
         project && title && starts_at
       end
@@ -105,6 +111,10 @@ module Gitlab
         Rack::Utils.parse_query(uri.query).fetch('g0.expr')
       rescue URI::InvalidURIError, KeyError
       end
+
+      def parse_alert_markdown_from_payload
+        payload&.dig('annotations', 'gitlab_incident_markdown')
+      end
     end
   end
 end

ee/spec/lib/gitlab/alerting/alert_spec.rb

@@ -189,6 +189,12 @@ describe Gitlab::Alerting::Alert do
     end
   end
 
+  describe '#alert_markdown' do
+    subject { alert.alert_markdown }
+
+    it_behaves_like 'parse payload', 'annotations/gitlab_incident_markdown'
+  end
+
   describe '#valid?' do
     before do
       payload.update(