Add Prometheus metrics to ActionCable subscription events

- Add changelog
- Add to metrics docs

changelogs/unreleased/296845-cablett-actioncable-prometheus-metrics.yml

+---
+title: Add Prometheus metrics for ActionCable subscription events
+merge_request: 56157
+author:
+type: added

config/initializers/zz_metrics.rb

@@ -143,6 +143,7 @@ if Gitlab::Metrics.enabled? && !Rails.env.test? && !(Rails.env.development? && d
 
   # These are manually require'd so the classes are registered properly with
   # ActiveSupport.
+  require_dependency 'gitlab/metrics/subscribers/action_cable'
   require_dependency 'gitlab/metrics/subscribers/action_view'
   require_dependency 'gitlab/metrics/subscribers/active_record'
   require_dependency 'gitlab/metrics/subscribers/rails_cache'

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -116,6 +116,10 @@ The following metrics are available:
 | `action_cable_pool_largest_size`                                 | Gauge       | 13.4    | Largest number of worker threads observed so far in ActionCable thread pool                                           | `server_mode`                                             |
 | `action_cable_pool_pending_tasks`                                | Gauge       | 13.4    | Number of tasks waiting to be executed in ActionCable thread pool                                                     | `server_mode`                                             |
 | `action_cable_pool_tasks_total`                                  | Gauge       | 13.4    | Total number of tasks executed in ActionCable thread pool                                                             | `server_mode`                                             |
+| `action_cable_subscription_confirmations_total`                  | Counter     | 13.10   | The number of ActionCable subscriptions from clients confirmed                                                        | `server_mode`                                             |
+| `action_cable_subscription_rejections_total`                     | Counter     | 13.10   | The number of ActionCable subscriptions from clients rejected                                                         | `server_mode`                                             |
+| `action_cable_single_client_transmissions_total`                 | Counter     | 13.10   | The number of ActionCable messages transmitted to any client in any channel                                           | `server_mode`                                             |
+| `action_cable_broadcasts_total`                                  | Counter     | 13.10   | The number of ActionCable broadcasts emitted                                                                          | `server_mode`                                             |
 | `gitlab_issuable_fast_count_by_state_total`                      | Counter     | 13.5    | Total number of row count operations on issue/merge request list pages                                                |                                                           |
 | `gitlab_issuable_fast_count_by_state_failures_total`             | Counter     | 13.5    | Number of soft-failed row count operations on issue/merge request list pages                                          |                                                           |
 | `gitlab_external_http_total`                                     | Counter     | 13.8    | Total number of HTTP calls to external systems                                                                        | `controller`, `action`                                    |

lib/gitlab/metrics/subscribers/action_cable.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Metrics
+    module Subscribers
+      class ActionCable < ActiveSupport::Subscriber
+        include Gitlab::Utils::StrongMemoize
+
+        attach_to :action_cable
+
+        SINGLE_CLIENT_TRANSMISSION = :action_cable_single_client_transmissions_total
+        TRANSMIT_SUBSCRIPTION_CONFIRMATION = :action_cable_subscription_confirmations_total
+        TRANSMIT_SUBSCRIPTION_REJECTION = :action_cable_subscription_rejections_total
+        BROADCAST = :action_cable_broadcasts_total
+
+        def transmit_subscription_confirmation(event)
+          confirm_subscription_counter.increment
+        end
+
+        def transmit_subscription_rejection(event)
+          reject_subscription_counter.increment
+        end
+
+        def transmit(event)
+          transmit_counter.increment
+        end
+
+        def broadcast(event)
+          broadcast_counter.increment
+        end
+
+        private
+
+        def transmit_counter
+          strong_memoize("transmission_counter") do
+            ::Gitlab::Metrics.counter(
+              SINGLE_CLIENT_TRANSMISSION,
+              'The number of ActionCable messages transmitted to any client in any channel'
+            )
+          end
+        end
+
+        def broadcast_counter
+          strong_memoize("broadcast_counter") do
+            ::Gitlab::Metrics.counter(
+              BROADCAST,
+              'The number of ActionCable broadcasts emitted'
+            )
+          end
+        end
+
+        def confirm_subscription_counter
+          strong_memoize("confirm_subscription_counter") do
+            ::Gitlab::Metrics.counter(
+              TRANSMIT_SUBSCRIPTION_CONFIRMATION,
+              'The number of ActionCable subscriptions from clients confirmed'
+            )
+          end
+        end
+
+        def reject_subscription_counter
+          strong_memoize("reject_subscription_counter") do
+            ::Gitlab::Metrics.counter(
+              TRANSMIT_SUBSCRIPTION_REJECTION,
+              'The number of ActionCable subscriptions from clients rejected'
+            )
+          end
+        end
+      end
+    end
+  end
+end

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

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Metrics::Subscribers::ActionCable, :request_store do
+  let(:subscriber) { described_class.new }
+  let(:data) { { data: { event: 'updated' } } }
+  let(:channel_class) { 'IssuesChannel' }
+  let(:event) do
+    double(
+      :event,
+      name: name,
+      payload: payload
+    )
+  end
+
+  describe '#transmit' do
+    let(:name) { 'transmit.action_cable' }
+    let(:via) { 'streamed from issues:Z2lkOi8vZs2l0bGFiL0lzc3VlLzQ0Ng' }
+    let(:payload) do
+      {
+        channel_class: channel_class,
+        via: via,
+        data: data
+      }
+    end
+
+    it 'tracks the transmit event' do
+      expect(::Gitlab::Metrics).to receive(:counter)
+        .with(Gitlab::Metrics::Subscribers::ActionCable::SINGLE_CLIENT_TRANSMISSION, /transmit/)
+        .and_call_original
+
+      subscriber.transmit(event)
+    end
+  end
+
+  describe '#broadcast' do
+    let(:name) { 'broadcast.action_cable' }
+    let(:coder) { ActiveSupport::JSON }
+    let(:message) do
+      { event: :updated }
+    end
+
+    let(:broadcasting) { 'issues:Z2lkOi8vZ2l0bGFiL0lzc3VlLzQ0Ng' }
+    let(:payload) do
+      {
+        broadcasting: broadcasting,
+        message: message,
+        coder: coder
+      }
+    end
+
+    it 'tracks the broadcast event' do
+      expect(::Gitlab::Metrics).to receive(:counter)
+        .with(Gitlab::Metrics::Subscribers::ActionCable::BROADCAST, /broadcast/)
+        .and_call_original
+
+      subscriber.broadcast(event)
+    end
+  end
+
+  describe '#transmit_subscription_confirmation' do
+    let(:name) { 'transmit_subscription_confirmation.action_cable' }
+    let(:channel_class) { 'IssuesChannel' }
+    let(:payload) do
+      {
+        channel_class: channel_class
+      }
+    end
+
+    it 'tracks the transmit event' do
+      expect(::Gitlab::Metrics).to receive(:counter)
+        .with(Gitlab::Metrics::Subscribers::ActionCable::TRANSMIT_SUBSCRIPTION_CONFIRMATION, /confirm/)
+        .and_call_original
+
+      subscriber.transmit_subscription_confirmation(event)
+    end
+  end
+
+  describe '#transmit_subscription_rejection' do
+    let(:name) { 'transmit_subscription_rejection.action_cable' }
+    let(:channel_class) { 'IssuesChannel' }
+    let(:payload) do
+      {
+          channel_class: channel_class
+      }
+    end
+
+    it 'tracks the transmit event' do
+      expect(::Gitlab::Metrics).to receive(:counter)
+        .with(Gitlab::Metrics::Subscribers::ActionCable::TRANSMIT_SUBSCRIPTION_REJECTION, /reject/)
+        .and_call_original
+
+      subscriber.transmit_subscription_rejection(event)
+    end
+  end
+end