Swap multiple_database_metrics FF for env var

This is because it is quite expensive to be reading this feature flag
every single request. Checking an env var is much more efficient because
feature flags are read from the database.

This feature flag was not user- or administrator-facing.

Changelog: other

config/feature_flags/development/multiple_database_metrics.yml

----
-name: multiple_database_metrics
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63490
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/333227
-milestone: '14.1'
-type: development
-group: group::sharding
-default_enabled: false

doc/administration/monitoring/performance/performance_bar.md

@@ -31,9 +31,9 @@ From left to right, the performance bar displays:
     was sent to the read/write primary server. "Replica" means it was sent
     to a read-only replica.
   - **Config name**: shows up only when the
-    `multiple_database_metrics` feature flag is enabled. This is used to
-    distinguish between different databases configured for different GitLab
-    features. The name shown is the same name used to configure database
+    `GITLAB_MULTIPLE_DATABASE_METRICS` environment variable is set. 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.
 - **Gitaly calls**: the time taken (in milliseconds) and the total number of
   [Gitaly](../../gitaly/index.md) calls. Click to display a modal window with more

lib/gitlab/metrics/subscribers/active_record.rb

@@ -175,7 +175,7 @@ module Gitlab
             DB_LOAD_BALANCING_ROLES.each do |role|
               metrics.each do |metric|
                 counters << compose_metric_key(metric, role)
-                next unless ::Feature.enabled?(:multiple_database_metrics, default_enabled: :yaml)
+                next unless ENV['GITLAB_MULTIPLE_DATABASE_METRICS']
 
                 ::Gitlab::Database.db_config_names.each do |config_name|
                   counters << compose_metric_key(metric, role, config_name)

lib/peek/views/active_record.rb

@@ -81,7 +81,7 @@ module Peek
       end
 
       def format_call_details(call)
-        if Feature.enabled?(:multiple_database_metrics, default_enabled: :yaml)
+        if ENV['GITLAB_MULTIPLE_DATABASE_METRICS']
           super
         else
           super.except(:db_config_name)

spec/lib/peek/views/active_record_spec.rb

@@ -109,9 +109,9 @@ RSpec.describe Peek::Views::ActiveRecord, :request_store do
       )
     end
 
-    context 'when the multiple_database_metrics feature flag is disabled' do
+    context 'when the GITLAB_MULTIPLE_DATABASE_METRICS env var is disabled' do
       before do
-        stub_feature_flags(multiple_database_metrics: false)
+        stub_env('GITLAB_MULTIPLE_DATABASE_METRICS', nil)
       end
 
       it 'does not include db_config_name field' do
@@ -191,9 +191,9 @@ RSpec.describe Peek::Views::ActiveRecord, :request_store do
       )
     end
 
-    context 'when the multiple_database_metrics feature flag is disabled' do
+    context 'when the GITLAB_MULTIPLE_DATABASE_METRICS env var is disabled' do
       before do
-        stub_feature_flags(multiple_database_metrics: false)
+        stub_env('GITLAB_MULTIPLE_DATABASE_METRICS', nil)
       end
 
       it 'does not include db_config_name field' do

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

@@ -64,9 +64,9 @@ RSpec.shared_examples 'store ActiveRecord info in RequestStore' do |db_role|
     end
   end
 
-  context 'when multiple_database_metrics is disabled' do
+  context 'when the GITLAB_MULTIPLE_DATABASE_METRICS env var is disabled' do
     before do
-      stub_feature_flags(multiple_database_metrics: false)
+      stub_env('GITLAB_MULTIPLE_DATABASE_METRICS', nil)
     end
 
     it 'does not include per database metrics' do