Remove `GITLAB_MULTIPLE_DATABASE_METRICS` env feature flag

This feature is rolled out everywhere now. It is safe to remove
it and ensure that `db_config_name` is present always.

doc/administration/monitoring/performance/performance_bar.md

@@ -30,8 +30,7 @@ From left to right, the performance bar displays:
     is enabled. It shows which server role was used for the query.
     "Primary" means that the query was sent to the read/write primary server.
     "Replica" means it was sent to a read-only replica.
-  - **Configuration name**: shows up only when the
-    `GITLAB_MULTIPLE_DATABASE_METRICS` environment variable is set. This is
+  - **Configuration name**: this is
     used to distinguish between different databases configured for different
     GitLab features. The name shown is the same name used to configure database
     connections in GitLab.

lib/gitlab/metrics/subscribers/active_record.rb

@@ -134,11 +134,7 @@ module Gitlab
                              :"gitlab_transaction_db_#{counter}_total"
                            end
 
-          if ENV['GITLAB_MULTIPLE_DATABASE_METRICS']
-            current_transaction&.increment(prometheus_key, 1, { db_config_name: db_config_name })
-          else
-            current_transaction&.increment(prometheus_key, 1)
-          end
+          current_transaction&.increment(prometheus_key, 1, { db_config_name: db_config_name })
 
           Gitlab::SafeRequestStore[log_key] = Gitlab::SafeRequestStore[log_key].to_i + 1
 
@@ -154,11 +150,7 @@ module Gitlab
         def observe(histogram, event, &block)
           db_config_name = db_config_name(event.payload)
 
-          if ENV['GITLAB_MULTIPLE_DATABASE_METRICS']
-            current_transaction&.observe(histogram, event.duration / 1000.0, { db_config_name: db_config_name }, &block)
-          else
-            current_transaction&.observe(histogram, event.duration / 1000.0, &block)
-          end
+          current_transaction&.observe(histogram, event.duration / 1000.0, { db_config_name: db_config_name }, &block)
         end
 
         def current_transaction
@@ -193,11 +185,9 @@ module Gitlab
               counters << compose_metric_key(metric, role)
             end
 
-            if ENV['GITLAB_MULTIPLE_DATABASE_METRICS']
-              ::Gitlab::Database.db_config_names.each do |config_name|
-                counters << compose_metric_key(metric, nil, config_name) # main
-                counters << compose_metric_key(metric, nil, config_name + ::Gitlab::Database::LoadBalancing::LoadBalancer::REPLICA_SUFFIX) # main_replica
-              end
+            ::Gitlab::Database.db_config_names.each do |config_name|
+              counters << compose_metric_key(metric, nil, config_name) # main
+              counters << compose_metric_key(metric, nil, config_name + ::Gitlab::Database::LoadBalancing::LoadBalancer::REPLICA_SUFFIX) # main_replica
             end
           end
 

lib/peek/views/active_record.rb

@@ -75,14 +75,6 @@ module Peek
 
         "Role: #{role.to_s.capitalize}"
       end
-
-      def format_call_details(call)
-        if ENV['GITLAB_MULTIPLE_DATABASE_METRICS']
-          super
-        else
-          super.except(:db_config_name)
-        end
-      end
     end
   end
 end

spec/lib/peek/views/active_record_spec.rb

@@ -119,16 +119,4 @@ RSpec.describe Peek::Views::ActiveRecord, :request_store do
       )
     )
   end
-
-  context 'when the GITLAB_MULTIPLE_DATABASE_METRICS env var is disabled' do
-    before do
-      stub_env('GITLAB_MULTIPLE_DATABASE_METRICS', nil)
-    end
-
-    it 'does not include db_config_name field' do
-      ActiveSupport::Notifications.publish('sql.active_record', Time.current, Time.current + 1.second, '1', event_1)
-
-      expect(subject.results[:details][0][:db_config_name]).to be_nil
-    end
-  end
 end

spec/support/enable_multiple_database_metrics_by_default.rb

-# frozen_string_literal: true
-
-RSpec.configure do |config|
-  config.before do
-    # Enable this by default in all tests so it behaves like a FF
-    stub_env('GITLAB_MULTIPLE_DATABASE_METRICS', '1')
-  end
-end

spec/support/shared_examples/metrics/active_record_subscriber_shared_examples.rb

@@ -91,21 +91,6 @@ RSpec.shared_examples 'store ActiveRecord info in RequestStore' do |db_role|
       end
     end
   end
-
-  context 'when the GITLAB_MULTIPLE_DATABASE_METRICS env var is disabled' do
-    before do
-      stub_env('GITLAB_MULTIPLE_DATABASE_METRICS', nil)
-    end
-
-    it 'does not include per database metrics' do
-      Gitlab::WithRequestStore.with_request_store do
-        subscriber.sql(event)
-
-        expect(described_class.db_counter_payload).not_to include(:"db_replica_#{db_config_name}_duration_s")
-        expect(described_class.db_counter_payload).not_to include(:"db_replica_#{db_config_name}_count")
-      end
-    end
-  end
 end
 
 RSpec.shared_examples 'record ActiveRecord metrics in a metrics transaction' do |db_role|
@@ -160,26 +145,6 @@ RSpec.shared_examples 'record ActiveRecord metrics in a metrics transaction' do
 
     subscriber.sql(event)
   end
-
-  context 'when the GITLAB_MULTIPLE_DATABASE_METRICS env var is disabled' do
-    before do
-      stub_env('GITLAB_MULTIPLE_DATABASE_METRICS', nil)
-    end
-
-    it 'does not include db_config_name label' do
-      allow(transaction).to receive(:increment) do |*args|
-        labels = args[2] || {}
-        expect(labels).not_to include(:db_config_name)
-      end
-
-      allow(transaction).to receive(:observe) do |*args|
-        labels = args[2] || {}
-        expect(labels).not_to include(:db_config_name)
-      end
-
-      subscriber.sql(event)
-    end
-  end
 end
 
 RSpec.shared_examples 'record ActiveRecord metrics' do |db_role|