Merge branch 'initialize-fewer-metrics' into 'master'

Only initialise metrics for the most common feature categories

See merge request gitlab-org/gitlab!46558

lib/gitlab/metrics/requests_rack_middleware.rb

@@ -18,6 +18,15 @@ module Gitlab
       FEATURE_CATEGORY_HEADER = 'X-Gitlab-Feature-Category'
       FEATURE_CATEGORY_DEFAULT = 'unknown'
 
+      # These were the top 5 categories at a point in time, chosen as a
+      # reasonable default. If we initialize every category we'll end up
+      # with an explosion in unused metric combinations, but we want the
+      # most common ones to be always present.
+      FEATURE_CATEGORIES_TO_INITIALIZE = ['authentication_and_authorization',
+                                          'code_review', 'continuous_integration',
+                                          'not_owned', 'source_code_management',
+                                          FEATURE_CATEGORY_DEFAULT].freeze
+
       def initialize(app)
         @app = app
       end
@@ -46,16 +55,10 @@ module Gitlab
         #
         # For example `rate(http_requests_total{status="500"}[1m])` would return
         # no data until the first 500 error would occur.
-
-        # The list of feature categories is currently not needed by the application
-        # anywhere else. So no need to keep these in memory forever.
-        # Doing it here, means we're only reading the file on boot.
-        feature_categories = YAML.load_file(Rails.root.join('config', 'feature_categories.yml')).map(&:strip).uniq << FEATURE_CATEGORY_DEFAULT
-
         HTTP_METHODS.each do |method, statuses|
           http_request_duration_seconds.get({ method: method })
 
-          statuses.product(feature_categories) do |status, feature_category|
+          statuses.product(FEATURE_CATEGORIES_TO_INITIALIZE) do |status, feature_category|
             http_requests_total.get({ method: method, status: status, feature_category: feature_category })
           end
         end

spec/lib/gitlab/metrics/requests_rack_middleware_spec.rb

@@ -129,12 +129,11 @@ RSpec.describe Gitlab::Metrics::RequestsRackMiddleware, :aggregate_failures do
 
     describe '.initialize_metrics', :prometheus do
       it "sets labels for http_requests_total" do
-        feature_categories = YAML.load_file(Rails.root.join('config', 'feature_categories.yml')).map(&:strip) << described_class::FEATURE_CATEGORY_DEFAULT
         expected_labels = []
 
         described_class::HTTP_METHODS.each do |method, statuses|
           statuses.each do |status|
-            feature_categories.each do |feature_category|
+            described_class::FEATURE_CATEGORIES_TO_INITIALIZE.each do |feature_category|
               expected_labels << { method: method.to_s, status: status.to_s, feature_category: feature_category.to_s }
             end
           end
@@ -152,6 +151,13 @@ RSpec.describe Gitlab::Metrics::RequestsRackMiddleware, :aggregate_failures do
 
         expect(described_class.http_request_duration_seconds.values.keys).to include(*expected_labels)
       end
+
+      it 'has every label in config/feature_categories.yml' do
+        defaults = [described_class::FEATURE_CATEGORY_DEFAULT, 'not_owned']
+        feature_categories = YAML.load_file(Rails.root.join('config', 'feature_categories.yml')).map(&:strip) + defaults
+
+        expect(described_class::FEATURE_CATEGORIES_TO_INITIALIZE).to all(be_in(feature_categories))
+      end
     end
   end
 end