Add a generic alert notification parser

As a prototype, the parser will turn initial payload into a Prometheus
specific format.
That helps us reuse the existing implementation of creating issues based
on Prometheus alerts.
Follow up changes will extend the parser to more fields.

ee/app/services/projects/alerting/notify_service.rb

@@ -27,7 +27,11 @@ module Projects
 
       def process_incident_issues
         IncidentManagement::ProcessAlertWorker
-          .perform_async(project.id, params.to_h)
+          .perform_async(project.id, parsed_payload)
+      end
+
+      def parsed_payload
+        Gitlab::Alerting::NotificationPayloadParser.call(params.to_h)
       end
     end
   end

ee/lib/gitlab/alerting/notification_payload_parser.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Alerting
+    class NotificationPayloadParser
+      def initialize(payload)
+        @payload = payload.to_h.with_indifferent_access
+      end
+
+      def self.call(payload)
+        new(payload).call
+      end
+
+      def call
+        {
+          'annotations' => {
+            'title' => payload[:title]
+          },
+          'startsAt' => payload[:starts_at]
+        }
+      end
+
+      private
+
+      attr_reader :payload
+    end
+  end
+end

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

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+describe Gitlab::Alerting::NotificationPayloadParser do
+  describe '.call' do
+    subject { described_class.call(payload) }
+
+    let(:starts_at) { Time.now.change(usec: 0) }
+    let(:payload) do
+      {
+        'title' => 'alert title',
+        'starts_at' => starts_at.rfc3339
+      }
+    end
+
+    it 'returns Prometheus-like payload' do
+      is_expected.to eq(
+        {
+          'annotations' => {
+            'title' => 'alert title'
+          },
+          'startsAt' => starts_at.rfc3339
+        }
+      )
+    end
+  end
+end

ee/spec/services/projects/alerting/notify_service_spec.rb

@@ -17,10 +17,8 @@ describe Projects::Alerting::NotifyService do
     let(:service) { described_class.new(project, nil, payload) }
     let(:payload_raw) do
       {
-        'annotations' => {
-          'title' => 'annotation title'
-        },
-        'startsAt' => starts_at.rfc3339
+        'title' => 'alert title',
+        'starts_at' => starts_at.rfc3339
       }
     end
     let(:payload) { ActionController::Parameters.new(payload_raw).permit! }