Track processed loose FK records

This change tracks the updated loose_foreign_keys_deleted_records
count during the cleanup process in Prometheus.

Changelog: added

app/services/loose_foreign_keys/batch_cleaner_service.rb

@@ -7,6 +7,10 @@ def initialize(parent_klass:, deleted_parent_records:, modification_tracker: Loo
       @deleted_parent_records = deleted_parent_records
       @modification_tracker = modification_tracker
       @models_by_table_name = models_by_table_name
+      @deleted_records_counter = Gitlab::Metrics.counter(
+        :loose_foreign_key_processed_deleted_records,
+        'The number of processed loose foreign key deleted records'
+      )
     end
 
     def execute
@@ -21,13 +25,15 @@ def execute
       return if modification_tracker.over_limit?
 
       # At this point, all associations are cleaned up, we can update the status of the parent records
-      LooseForeignKeys::DeletedRecord
+      update_count = LooseForeignKeys::DeletedRecord
         .mark_records_processed_for_table_between(deleted_parent_records.first.fully_qualified_table_name, deleted_parent_records.first, deleted_parent_records.last)
+
+      deleted_records_counter.increment({ table: parent_klass.table_name, db_config_name: LooseForeignKeys::DeletedRecord.connection.pool.db_config.name }, update_count)
     end
 
     private
 
-    attr_reader :parent_klass, :deleted_parent_records, :modification_tracker, :models_by_table_name
+    attr_reader :parent_klass, :deleted_parent_records, :modification_tracker, :models_by_table_name, :deleted_records_counter
 
     def record_result(cleaner, result)
       if cleaner.async_delete?

spec/services/loose_foreign_keys/batch_cleaner_service_spec.rb

@@ -79,6 +79,8 @@ def create_table_structure
   end
 
   context 'when parent records are deleted' do
+    let(:deleted_records_counter) { Gitlab::Metrics.registry.get(:loose_foreign_key_processed_deleted_records) }
+
     before do
       parent_record_1.delete
 
@@ -98,8 +100,15 @@ def create_table_structure
       expect(loose_fk_child_table_2.where(parent_id_with_different_column: nil).count).to eq(2)
     end
 
-    it 'cleans up the parent DeletedRecord' do
+    it 'cleans up the pending parent DeletedRecord' do
       expect(LooseForeignKeys::DeletedRecord.status_pending.count).to eq(0)
+      expect(LooseForeignKeys::DeletedRecord.status_processed.count).to eq(1)
+    end
+
+    it 'records the DeletedRecord status updates', :prometheus do
+      counter = Gitlab::Metrics.registry.get(:loose_foreign_key_processed_deleted_records)
+
+      expect(counter.get(table: parent_model.table_name, db_config_name: 'main')).to eq(1)
     end
 
     it 'does not delete unrelated records' do