Implement a subscriber for per-request cache usage of RackAttack

Issue https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/751

lib/gitlab/metrics/subscribers/rack_attack.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Metrics
+    module Subscribers
+      # Instrument the cache operations of RackAttack to use in structured
+      # logs. Two fields are exposed:
+      # - rack_attack_redis_count: the number of redis calls triggered by
+      # RackAttack in a request.
+      # - rack_attack_redis_duration_s: the total duration of all redis calls
+      # triggered by RackAttack in a request.
+      class RackAttack < ActiveSupport::Subscriber
+        INSTRUMENTATION_STORE_KEY = :rack_attack_instrumentation
+        attach_to 'redis'
+
+        PAYLOAD_KEYS = [
+          :rack_attack_redis_count,
+          :rack_attack_redis_duration_s
+        ].freeze
+
+        def self.payload
+          Gitlab::SafeRequestStore[INSTRUMENTATION_STORE_KEY] ||= {
+            rack_attack_redis_count: 0,
+            rack_attack_redis_duration_s: 0.0
+          }
+        end
+
+        def rack_attack(event)
+          self.class.payload[:rack_attack_redis_count] += 1
+          self.class.payload[:rack_attack_redis_duration_s] += event.duration.to_f / 1000
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/metrics/subscribers/rack_attack_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Metrics::Subscribers::RackAttack, :request_store do
+  let(:subscriber) { described_class.new }
+
+  describe '.payload' do
+    context 'when the request store is empty' do
+      it 'returns empty data' do
+        expect(described_class.payload).to eql(
+          rack_attack_redis_count: 0,
+          rack_attack_redis_duration_s: 0.0
+        )
+      end
+    end
+
+    context 'when the request store already has data' do
+      before do
+        Gitlab::SafeRequestStore[:rack_attack_instrumentation] = {
+          rack_attack_redis_count: 10,
+          rack_attack_redis_duration_s: 9.0
+        }
+      end
+
+      it 'returns the accumulated data' do
+        expect(described_class.payload).to eql(
+          rack_attack_redis_count: 10,
+          rack_attack_redis_duration_s: 9.0
+        )
+      end
+    end
+  end
+
+  describe '#rack_attack' do
+    it 'accumulates per-request RackAttack cache usage' do
+      freeze_time do
+        subscriber.rack_attack(
+          ActiveSupport::Notifications::Event.new(
+            'rack_attack.redis', Time.current, Time.current + 1.second, '1', { operation: 'fetch' }
+          )
+        )
+        subscriber.rack_attack(
+          ActiveSupport::Notifications::Event.new(
+            'rack_attack.redis', Time.current, Time.current + 2.seconds, '1', { operation: 'write' }
+          )
+        )
+        subscriber.rack_attack(
+          ActiveSupport::Notifications::Event.new(
+            'rack_attack.redis', Time.current, Time.current + 3.seconds, '1', { operation: 'read' }
+          )
+        )
+      end
+
+      expect(Gitlab::SafeRequestStore[:rack_attack_instrumentation]).to eql(
+        rack_attack_redis_count: 3,
+        rack_attack_redis_duration_s: 6.0
+      )
+    end
+  end
+end