Add feature category label to http_request_total counter

As this counter is implemented in a Rack middleware, the easiest way to
exfiltrate the category is just to add it to a header.

app/controllers/application_controller.rb

@@ -271,6 +271,7 @@ class ApplicationController < ActionController::Base
     headers['X-XSS-Protection'] = '1; mode=block'
     headers['X-UA-Compatible'] = 'IE=edge'
     headers['X-Content-Type-Options'] = 'nosniff'
+    headers[Gitlab::Metrics::RequestsRackMiddleware::FEATURE_CATEGORY_HEADER] = feature_category
   end
 
   def default_cache_headers
@@ -551,6 +552,10 @@ class ApplicationController < ActionController::Base
     "#{self.class.name}##{action_name}"
   end
 
+  def feature_category
+    self.class.feature_category_for_action(action_name).to_s
+  end
+
   def required_signup_info
     return unless current_user
     return unless current_user.role_required?

lib/gitlab/metrics/requests_rack_middleware.rb

@@ -15,6 +15,9 @@ module Gitlab
 
       HEALTH_ENDPOINT = /^\/-\/(liveness|readiness|health|metrics)\/?$/.freeze
 
+      FEATURE_CATEGORY_HEADER = 'X-Gitlab-Feature-Category'
+      FEATURE_CATEGORY_DEFAULT = 'unknown'
+
       def initialize(app)
         @app = app
       end
@@ -50,11 +53,13 @@ module Gitlab
         started = Time.now.to_f
         health_endpoint = health_endpoint?(env['PATH_INFO'])
         status = 'undefined'
+        feature_category = nil
 
         begin
           status, headers, body = @app.call(env)
 
           elapsed = Time.now.to_f - started
+          feature_category = headers&.fetch(FEATURE_CATEGORY_HEADER, nil)
 
           unless health_endpoint
             RequestsRackMiddleware.http_request_duration_seconds.observe({ method: method, status: status.to_s }, elapsed)
@@ -66,9 +71,9 @@ module Gitlab
           raise
         ensure
           if health_endpoint
-            RequestsRackMiddleware.http_health_requests_total.increment(method: method, status: status)
+            RequestsRackMiddleware.http_health_requests_total.increment(status: status, method: method)
           else
-            RequestsRackMiddleware.http_request_total.increment(method: method, status: status)
+            RequestsRackMiddleware.http_request_total.increment(status: status, method: method, feature_category: feature_category || FEATURE_CATEGORY_DEFAULT)
           end
         end
       end

spec/lib/gitlab/metrics/requests_rack_middleware_spec.rb

@@ -22,7 +22,7 @@ RSpec.describe Gitlab::Metrics::RequestsRackMiddleware do
       end
 
       it 'increments requests count' do
-        expect(described_class).to receive_message_chain(:http_request_total, :increment).with(method: 'get', status: 200)
+        expect(described_class).to receive_message_chain(:http_request_total, :increment).with(method: 'get', status: 200, feature_category: 'unknown')
 
         subject.call(env)
       end
@@ -68,7 +68,7 @@ RSpec.describe Gitlab::Metrics::RequestsRackMiddleware do
             end
 
             it 'increments overall counter rather than health endpoint counter' do
-              expect(described_class).to receive_message_chain(:http_request_total, :increment).with(method: 'get', status: 200)
+              expect(described_class).to receive_message_chain(:http_request_total, :increment).with(method: 'get', status: 200, feature_category: 'unknown')
               expect(described_class).not_to receive(:http_health_requests_total)
 
               subject.call(env)
@@ -101,7 +101,7 @@ RSpec.describe Gitlab::Metrics::RequestsRackMiddleware do
       end
 
       it 'increments requests count' do
-        expect(described_class).to receive_message_chain(:http_request_total, :increment).with(method: 'get', status: 'undefined')
+        expect(described_class).to receive_message_chain(:http_request_total, :increment).with(method: 'get', status: 'undefined', feature_category: 'unknown')
 
         expect { subject.call(env) }.to raise_error(StandardError)
       end
@@ -113,6 +113,27 @@ RSpec.describe Gitlab::Metrics::RequestsRackMiddleware do
       end
     end
 
+    context 'when a feature category header is present' do
+      before do
+        allow(app).to receive(:call).and_return([200, { described_class::FEATURE_CATEGORY_HEADER => 'issue_tracking' }, nil])
+      end
+
+      it 'adds the feature category to the labels for http_request_total' do
+        expect(described_class).to receive_message_chain(:http_request_total, :increment).with(method: 'get', status: 200, feature_category: 'issue_tracking')
+
+        subject.call(env)
+      end
+
+      it 'does not record a feature category for health check endpoints' do
+        env['PATH_INFO'] = '/-/liveness'
+
+        expect(described_class).to receive_message_chain(:http_health_requests_total, :increment).with(method: 'get', status: 200)
+        expect(described_class).not_to receive(:http_request_total)
+
+        subject.call(env)
+      end
+    end
+
     describe '.initialize_http_request_duration_seconds' do
       it "sets labels" do
         expected_labels = []