Move most of MetricsController logic to MetricsService

app/controllers/metrics_controller.rb

-require 'prometheus/client/formats/text'
-
 class MetricsController < ActionController::Base
   protect_from_forgery with: :exception
+  before_action :validate_prometheus_metrics
   include RequiresHealthToken
-  before_action :ensure_prometheus_metrics_are_enabled
-
-  CHECKS = [
-    Gitlab::HealthChecks::DbCheck,
-    Gitlab::HealthChecks::RedisCheck,
-    Gitlab::HealthChecks::FsShardsCheck
-  ].freeze
-
 
   def metrics
-    metrics_text = Prometheus::Client::Formats::Text.marshal_multiprocess(multiprocess_metrics_path)
-    response = "#{health_metrics_text}\n#{metrics_text}"
+    response = "#{metrics_service.health_metrics_text}\n#{metrics_service.prometheus_metrics_text}"
 
     render text: response, content_type: 'text/plain; version=0.0.4'
   end
 
   private
 
-  def ensure_prometheus_metrics_are_enabled
-    return render_404 unless Gitlab::Metrics.prometheus_metrics_enabled?
-  end
-
-  def multiprocess_metrics_path
-    Rails.root.join(ENV['prometheus_multiproc_dir'])
+  def metrics_service
+    @metrics_service ||= MetricsService.new
   end
 
-  def health_metrics_text
-    results = CHECKS.flat_map(&:metrics)
-
-    types = results.map(&:name).uniq.map { |metric_name| "# TYPE #{metric_name} gauge" }
-    metrics = results.map(&method(:metric_to_prom_line))
-
-    types.concat(metrics).join("\n")
-  end
-
-  def metric_to_prom_line(metric)
-    labels = metric.labels&.map { |key, value| "#{key}=\"#{value}\"" }&.join(',') || ''
-
-    if labels.empty?
-      "#{metric.name} #{metric.value}"
-    else
-      "#{metric.name}{#{labels}} #{metric.value}"
-    end
+  def validate_prometheus_metrics
+    render_404 unless Gitlab::Metrics.prometheus_metrics_enabled?
   end
 end

app/services/metrics_service.rb

+require 'prometheus/client/formats/text'
+
+class MetricsService
+  CHECKS = [
+    Gitlab::HealthChecks::DbCheck,
+    Gitlab::HealthChecks::RedisCheck,
+    Gitlab::HealthChecks::FsShardsCheck
+  ].freeze
+
+  def prometheus_metrics_text
+    Prometheus::Client::Formats::Text.marshal_multiprocess(multiprocess_metrics_path)
+  end
+
+  def health_metrics_text
+    results = CHECKS.flat_map(&:metrics)
+
+    types = results.map(&:name).uniq.map { |metric_name| "# TYPE #{metric_name} gauge" }
+    metrics = results.map(&method(:metric_to_prom_line))
+
+    types.concat(metrics).join("\n")
+  end
+
+  private
+
+  def multiprocess_metrics_path
+    Rails.root.join(ENV['prometheus_multiproc_dir'])
+  end
+
+  def metric_to_prom_line(metric)
+    labels = metric.labels&.map { |key, value| "#{key}=\"#{value}\"" }&.join(',') || ''
+
+    if labels.empty?
+      "#{metric.name} #{metric.value}"
+    else
+      "#{metric.name}{#{labels}} #{metric.value}"
+    end
+  end
+end

spec/controllers/metrics_controller_spec.rb

@@ -50,7 +50,9 @@ describe MetricsController do
       end
 
       context 'prometheus metrics are disabled' do
-        allow(Gitlab::Metrics).to receive(:prometheus_metrics_enabled?).and_return(false)
+        before do
+          allow(Gitlab::Metrics).to receive(:prometheus_metrics_enabled?).and_return(false)
+        end
 
         it 'returns proper response' do
           get :metrics
@@ -65,6 +67,5 @@ describe MetricsController do
         expect(response.status).to eq(404)
       end
     end
-
   end
 end