Add Prometheus metrics for loose foreign keys

This extends ModificationTracker to keep track of updates and deletions
in Prometheus, in addition to the existing metrics.

Changelog: added

app/models/loose_foreign_keys/modification_tracker.rb

@@ -12,14 +12,24 @@ module LooseForeignKeys
       @delete_count_by_table = Hash.new { |h, k| h[k] = 0 }
       @update_count_by_table = Hash.new { |h, k| h[k] = 0 }
       @start_time = monotonic_time
+      @deletes_counter = Gitlab::Metrics.counter(
+        :loose_foreign_key_deletions,
+        'The number of loose foreign key deletions'
+      )
+      @updates_counter = Gitlab::Metrics.counter(
+        :loose_foreign_key_updates,
+        'The number of loose foreign key updates'
+      )
     end
 
     def add_deletions(table, count)
       @delete_count_by_table[table] += count
+      @deletes_counter.increment({ table: table }, count)
     end
 
     def add_updates(table, count)
       @update_count_by_table[table] += count
+      @updates_counter.increment({ table: table }, count)
     end
 
     def over_limit?

spec/models/loose_foreign_keys/modification_tracker_spec.rb

@@ -50,6 +50,26 @@ RSpec.describe LooseForeignKeys::ModificationTracker do
     end
   end
 
+  describe '#add_deletions' do
+    it 'increments a Prometheus counter' do
+      counter = Gitlab::Metrics.registry.get(:loose_foreign_key_deletions)
+
+      subject.add_deletions(:users, 4)
+
+      expect(counter.get(table: :users)).to eq(4)
+    end
+  end
+
+  describe '#add_updates' do
+    it 'increments a Prometheus counter' do
+      counter = Gitlab::Metrics.registry.get(:loose_foreign_key_updates)
+
+      subject.add_updates(:users, 4)
+
+      expect(counter.get(table: :users)).to eq(4)
+    end
+  end
+
   describe '#stats' do
     it 'exposes stats' do
       freeze_time do