Add feature flag for reindexing

config/feature_flags/ops/database_reindexing.yml

+---
+name: database_reindexing
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42705
+rollout_issue_url:
+group: group::database
+type: ops
+default_enabled: false
\ No newline at end of file

lib/tasks/gitlab/db.rake

@@ -169,6 +169,11 @@ namespace :gitlab do
 
     desc 'reindex a regular (non-unique) index without downtime to eliminate bloat'
     task :reindex, [:index_name] => :environment do |_, args|
+      unless Feature.enabled?(:database_reindexing, type: :ops)
+        puts "This feature (database_reindexing) is currently disabled.".yellow
+        exit
+      end
+
       raise ArgumentError, 'must give the index name to reindex' unless args[:index_name]
 
       Gitlab::Database::ConcurrentReindex.new(args[:index_name], logger: Logger.new(STDOUT)).execute