Add Prometheus ActionCableSampler

This will sample AC metrics periodically so they can be exposed
to Prometheus.

changelogs/unreleased/217314-actioncable-prometheus.yml

+---
+title: Export ActionCable metrics to Prometheus
+merge_request: 41358
+author:
+type: added

config/initializers/7_prometheus_metrics.rb

@@ -46,6 +46,10 @@ if !Rails.env.test? && Gitlab::Metrics.prometheus_metrics_enabled?
     Gitlab::Metrics::Samplers::DatabaseSampler.initialize_instance.start
     Gitlab::Metrics::Samplers::ThreadsSampler.initialize_instance.start
 
+    if Gitlab::Runtime.action_cable?
+      Gitlab::Metrics::Samplers::ActionCableSampler.instance.start
+    end
+
     if Gitlab.ee? && Gitlab::Runtime.sidekiq?
       Gitlab::Metrics::Samplers::GlobalSearchSampler.instance.start
     end

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -106,6 +106,7 @@ The following metrics are available:
 | `successful_login_captcha_total`                               | Gauge     |                   11.0 | Counter of successful CAPTCHA attempts during login                                                 |                                                     |
 | `auto_devops_pipelines_completed_total`                        | Counter   |                   12.7 | Counter of completed Auto DevOps pipelines, labeled by status                                       |                                                     |
 | `gitlab_metrics_dashboard_processing_time_ms`                  | Summary   |                  12.10 | Metrics dashboard processing time in milliseconds | service, stages |
+| `action_cable_active_connections`                              | Gauge     |                   13.4 | Number of ActionCable WS clients currently connected | `server_mode` |
 
 ## Metrics controlled by a feature flag
 

lib/gitlab/metrics/samplers/action_cable_sampler.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Metrics
+    module Samplers
+      class ActionCableSampler < BaseSampler
+        SAMPLING_INTERVAL_SECONDS = 5
+
+        def metrics
+          @metrics ||= {
+            active_connections: ::Gitlab::Metrics.gauge(
+              :action_cable_active_connections, 'Number of ActionCable WS clients currently connected'
+            )
+          }
+        end
+
+        def sample
+          stats = sample_stats
+          labels = {
+            server_mode: server_mode
+          }
+
+          metrics[:active_connections].set(labels, stats[:active_connections])
+        end
+
+        private
+
+        def sample_stats
+          {
+            active_connections: ::ActionCable.server.connections.size
+          }
+        end
+
+        def server_mode
+          Gitlab::ActionCable::Config.in_app? ? 'in-app' : 'standalone'
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/metrics/samplers/action_cable_sampler_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Metrics::Samplers::ActionCableSampler do
+  subject { described_class.new }
+
+  describe '#interval' do
+    it 'samples every five seconds by default' do
+      expect(subject.interval).to eq(5)
+    end
+
+    it 'samples at other intervals if requested' do
+      expect(described_class.new(11).interval).to eq(11)
+    end
+  end
+
+  describe '#sample' do
+    before do
+      expect(::ActionCable.server.connections).to receive(:size).and_return(42)
+    end
+
+    context 'for in-app mode' do
+      it 'samples statistic with correct labels attached' do
+        expect(Gitlab::ActionCable::Config).to receive(:in_app?).and_return(true)
+
+        expect(subject.metrics[:active_connections]).to receive(:set).with({ server_mode: 'in-app' }, 42)
+
+        subject.sample
+      end
+    end
+
+    context 'for standalone mode' do
+      it 'samples statistic with correct labels attached' do
+        expect(Gitlab::ActionCable::Config).to receive(:in_app?).and_return(false)
+
+        expect(subject.metrics[:active_connections]).to receive(:set).with({ server_mode: 'standalone' }, 42)
+
+        subject.sample
+      end
+    end
+  end
+end