Add last update at gauge

This allows us to reason about which gauge value to use when gauges are
being reported from multiple (sidekiq) hosts over time.

lib/gitlab/database/background_migration/batched_migration_wrapper.rb

@@ -67,6 +67,7 @@ module Gitlab
           metric_for(:counter_updated_tuples).increment(base_labels, tracking_record.batch_size)
           metric_for(:gauge_migrated_tuples).set(base_labels, tracking_record.batched_migration.migrated_tuple_count)
           metric_for(:gauge_total_tuple_count).set(base_labels, tracking_record.batched_migration.total_tuple_count)
+          metric_for(:gauge_last_update_time).set(base_labels, Time.current.to_i)
 
           if metrics = tracking_record.metrics
             metrics['timings']&.each do |key, timings|
@@ -120,6 +121,10 @@ module Gitlab
               gauge_total_tuple_count: Gitlab::Metrics.gauge(
                 :batched_migration_total_tuple_count,
                 'Total tuple count the migration needs to touch'
+              ),
+              gauge_last_update_time: Gitlab::Metrics.gauge(
+                :batched_migration_last_update_time_seconds,
+                'Unix epoch time in seconds'
               )
             }
           end

spec/lib/gitlab/database/background_migration/batched_migration_wrapper_spec.rb

@@ -142,7 +142,7 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigrationWrapper, '
     it 'reports job duration' do
       freeze_time do
         expect(Time).to receive(:current).and_return(Time.zone.now - 5.seconds).ordered
-        expect(Time).to receive(:current).and_return(Time.zone.now).ordered
+        allow(Time).to receive(:current).and_call_original
 
         expect(described_class.metrics[:gauge_job_duration]).to receive(:set).with(labels, 5.seconds)
 
@@ -155,6 +155,14 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigrationWrapper, '
 
       subject
     end
+
+    it 'reports last updated at timestamp' do
+      freeze_time do
+        expect(described_class.metrics[:gauge_last_update_time]).to receive(:set).with(labels, Time.current.to_i)
+
+        subject
+      end
+    end
   end
 
   context 'when the migration job does not raise an error' do