Fix N+1 queries in vulnerability exports

What does this MR do and why?

Following multiple reports of slowness, I did some profiling of VulnerabilityExports::Exporters::CsvService and found multiple N+1 queries. We can see that for each row, we are spending a lot of time in vulnerability.full_path, vulnerability.group_name, vulnerability.notes_summary, and vulnerability.project_name, and there are hits to database classes inside these nodes. This indicates that we have N+1 queries when reading these fields.

This MR fixes them by using preloading.

Relates to: #435358 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Seed a project with many vulnerabilities by modifying the vulnerabilities seeder and running it on a project with bin/rake 'gitlab:seed:vulnerabilities[my/project]':

   diff --git a/ee/lib/quality/seeders/vulnerabilities.rb b/ee/lib/quality/seeders/vulnerabilities.rb
   index ddc1bb3c49b2..4399f3a174c1 100644
   --- a/ee/lib/quality/seeders/vulnerabilities.rb
   +++ b/ee/lib/quality/seeders/vulnerabilities.rb
   @@ -14,7 +14,7 @@ def seed!
              return
            end
   
   -        30.times do |rank|
   +        10_000.times do |rank|
              primary_identifier = create_identifier(rank)
              vulnerability = create_vulnerability
              occurrence = create_occurrence(vulnerability, rank, primary_identifier)
   @@ -167,7 +167,7 @@ def pipeline
          end
   
          def author
   -        @author ||= project.users.first
   +        @author ||= User.find_by_username('bwill')
          end
        end
      end

2. Modify VulnerabilityExports::ExportService to record the export time:

   diff --git a/ee/app/services/vulnerability_exports/export_service.rb b/ee/app/services/vulnerability_exports/export_service.rb
   index 7a3c5ab3ce25..3768fe02cca6 100644
   --- a/ee/app/services/vulnerability_exports/export_service.rb
   +++ b/ee/app/services/vulnerability_exports/export_service.rb
   @@ -39,6 +39,13 @@ def generate_export
         vulnerability_export.start!
         generate_export_file
         vulnerability_export.finish!
   +
   +      file = File.new('export.out', 'w')
   +      file.write("Started at: #{vulnerability_export.started_at}\n")
   +      file.write("Finished at: #{vulnerability_export.finished_at}\n")
   +      time_taken = (vulnerability_export.finished_at - vulnerability_export.started_at) / 1.minute
   +      file.write("Time taken: #{time_taken} minutes\n")
   +      file.close
       rescue StandardError
         vulnerability_export.reset_state!
         raise

3. Go to <gdk_url>/my/project/-/security/vulnerability_report and click the export button.

4. Check export.out to see the time taken.

Testing this on a project with 8173 vulnerabilities yielded the following results, showing a 7x speed up.

Before:

Started at: 2023-12-14 20:25:37 UTC
Finished at: 2023-12-14 20:26:18 UTC
Time taken: 0.68087285 minutes

After:

Started at: 2023-12-14 20:19:18 UTC
Finished at: 2023-12-14 20:19:24 UTC
Time taken: 0.09568628333333334 minutes

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.