Return current time when start_time is missing

When the user of the endpoint didn't pass `start_time`,
then the parser will set it to the current time in RFC3339 format.

ee/lib/gitlab/alerting/notification_payload_parser.rb

@@ -16,7 +16,7 @@ module Gitlab
       def call
         {
           'annotations' => annotations,
-          'startsAt' => payload[:start_time]
+          'startsAt' => starts_at
         }.compact
       end
 
@@ -41,6 +41,14 @@ module Gitlab
       def hosts
         payload[:hosts] && Array(payload[:hosts])
       end
+
+      def starts_at
+        payload[:start_time].presence || current_time
+      end
+
+      def current_time
+        Time.now.change(usec: 0).rfc3339
+      end
     end
   end
 end

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

@@ -56,9 +56,10 @@ describe Gitlab::Alerting::NotificationPayloadParser do
     context 'when payload is blank' do
       let(:payload) { {} }
 
-      it 'returns only default title' do
+      it 'returns default parameters' do
         is_expected.to eq(
-          'annotations' => { 'title' => 'New: Incident' }
+          'annotations' => { 'title' => 'New: Incident' },
+          'startsAt' => starts_at.rfc3339
         )
       end
     end