Add started_at/completed_at timestamps to Elasticsearch migrations

When saving Elasticsearch migrations we set a started_at and
completed_at value. This will help us long term with debugging as this
information will make it much easier to find relevant logs for what
happened.

As this index was not declared with [strict dynamic
mapping](https://www.elastic.co/guide/en/elasticsearch/reference/7.x/dynamic.html)
it is totally safe to just add new fields to the payload without them
existing already in the configuration and I have tested locally that
this works as well.

For the sake of clarity, however, we have added them to the mappings as
there is no reason not to.

ee/app/models/elastic/migration_record.rb

@@ -16,7 +16,9 @@ module Elastic
     def save!(completed:)
       raise 'Migrations index is not found' unless helper.index_exists?(index_name: index_name)
 
-      client.index index: index_name, type: '_doc', id: version, body: { completed: completed }
+      data = { completed: completed }.merge(timestamps(completed: completed))
+
+      client.index index: index_name, type: '_doc', id: version, body: data
     end
 
     def persisted?
@@ -41,6 +43,15 @@ module Elastic
 
     private
 
+    def timestamps(completed:)
+      {}.tap do |data|
+        existing_data = load_from_index
+        data[:started_at] = existing_data&.dig('_source', 'started_at') || Time.now.utc
+
+        data[:completed_at] = Time.now.utc if completed
+      end
+    end
+
     def migration
       @migration ||= load_migration
     end

ee/lib/gitlab/elastic/helper.rb

@@ -63,6 +63,12 @@ module Gitlab
             properties: {
               completed: {
                 type: 'boolean'
+              },
+              started_at: {
+                type: 'date'
+              },
+              completed_at: {
+                type: 'date'
               }
             }
           }

ee/spec/models/elastic/migration_record_spec.rb

@@ -11,6 +11,34 @@ RSpec.describe Elastic::MigrationRecord, :elastic do
 
       expect { record.save!(completed: true) }.to raise_error(/index is not found/)
     end
+
+    it 'sets the started_at' do
+      record.save!(completed: false)
+
+      expect(record.load_from_index.dig('_source', 'started_at')).not_to be_nil
+    end
+
+    it 'does not update started_at on subsequent saves' do
+      record.save!(completed: false)
+
+      real_started_at = record.load_from_index.dig('_source', 'started_at')
+
+      record.save!(completed: false)
+
+      expect(record.load_from_index.dig('_source', 'started_at')).to eq(real_started_at)
+    end
+
+    it 'sets completed_at when completed' do
+      record.save!(completed: true)
+
+      expect(record.load_from_index.dig('_source', 'completed_at')).not_to be_nil
+    end
+
+    it 'does not set completed_at when not completed' do
+      record.save!(completed: false)
+
+      expect(record.load_from_index.dig('_source', 'completed_at')).to be_nil
+    end
   end
 
   describe '#persisted?' do