DB testing: Artifact full query log for each migration

All files placed in tmp/migration-testing/ are saved as pipeline
artifacts

lib/gitlab/database/migrations/observers.rb

@@ -7,7 +7,8 @@ module Gitlab
         def self.all_observers
           [
             TotalDatabaseSizeChange.new,
-            QueryStatistics.new
+            QueryStatistics.new,
+            QueryLog.new
           ]
         end
       end

lib/gitlab/database/migrations/observers/query_log.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module Migrations
+      module Observers
+        class QueryLog < MigrationObserver
+          def before
+            @logger_was = ActiveRecord::Base.logger
+            @log_file_path = File.join(Instrumentation::RESULT_DIR, 'current.log')
+            @logger = Logger.new(@log_file_path)
+            ActiveRecord::Base.logger = @logger
+          end
+
+          def after
+            ActiveRecord::Base.logger = @logger_was
+            @logger.close
+          end
+
+          def record(observation)
+            File.rename(@log_file_path, File.join(Instrumentation::RESULT_DIR, "#{observation.migration}.log"))
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/database/migrations/observers/query_log_spec.rb

+# frozen_string_literal: true
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::Migrations::Observers::QueryLog do
+  subject { described_class.new }
+
+  let(:observation) { Gitlab::Database::Migrations::Observation.new(migration) }
+  let(:connection) { ActiveRecord::Base.connection }
+  let(:query) { 'select 1' }
+  let(:directory_path) { Dir.mktmpdir }
+  let(:log_file) { "#{directory_path}/current.log" }
+  let(:migration) { 20210422152437 }
+
+  before do
+    stub_const('Gitlab::Database::Migrations::Instrumentation::RESULT_DIR', directory_path)
+  end
+
+  after do
+    FileUtils.remove_entry(directory_path)
+  end
+
+  it 'writes a file with the query log' do
+    observe
+
+    expect(File.read("#{directory_path}/#{migration}.log")).to include(query)
+  end
+
+  it 'does not change the default logger' do
+    expect { observe }.not_to change { ActiveRecord::Base.logger }
+  end
+
+  def observe
+    subject.before
+    connection.execute(query)
+    subject.after
+    subject.record(observation)
+  end
+end