Use Singleton module

This ensures that always only a single instance
of a given class can be created.

lib/gitlab/database/query_analyzer.rb

@@ -5,16 +5,14 @@ module Gitlab
     # The purpose of this class is to implement a various query analyzers based on `pg_query`
     # And process them all via `Gitlab::Database::QueryAnalyzers::*`
     class QueryAnalyzer
+      include ::Singleton
+
       ANALYZERS = [].freeze
 
       Parsed = Struct.new(
         :sql, :connection, :pg
       )
 
-      def self.instance
-        @instance ||= new
-      end
-
       def hook!
         @subscriber = ActiveSupport::Notifications.subscribe('sql.active_record') do |event|
           process_sql(event.payload[:sql], event.payload[:connection])

spec/lib/gitlab/database/query_analyzer_spec.rb

@@ -65,7 +65,7 @@ RSpec.describe Gitlab::Database::QueryAnalyzer do
 
     def process_sql(sql)
       ApplicationRecord.load_balancer.read_write do |connection|
-        described_class.new.send(:process_sql, sql, connection)
+        described_class.instance.send(:process_sql, sql, connection)
       end
     end
   end