Add mail-smtp_pool gem and update sample config

Updates the sample config to contain an example for enabling SMTP pool
support

Gemfile

@@ -511,6 +511,7 @@ gem 'erubi', '~> 1.9.0'
 # Monkey-patched in `config/initializers/mail_encoding_patch.rb`
 # See https://gitlab.com/gitlab-org/gitlab/issues/197386
 gem 'mail', '= 2.7.1'
+gem 'mail-smtp_pool', '~> 0.1.0', path: 'vendor/gems/mail-smtp_pool', require: false
 
 # File encryption
 gem 'lockbox', '~> 0.6.2'

Gemfile.lock

+PATH
+  remote: vendor/gems/mail-smtp_pool
+  specs:
+    mail-smtp_pool (0.1.0)
+      connection_pool (~> 2.0)
+      mail (~> 2.7)
+
 PATH
   remote: vendor/shims/mimemagic
   specs:
@@ -1483,6 +1490,7 @@ DEPENDENCIES
   loofah (~> 2.2)
   lru_redux
   mail (= 2.7.1)
+  mail-smtp_pool (~> 0.1.0)!
   marginalia (~> 1.10.0)
   memory_profiler (~> 0.9)
   method_source (~> 1.0)

changelogs/unreleased/230717-add-mail-smtp-pool-gem.yml

+---
+title: Add support for SMTP connection pooling when sending emails
+merge_request: 57805
+author:
+type: added

config/initializers/smtp_settings.rb.sample

@@ -22,3 +22,28 @@ if Rails.env.production?
     openssl_verify_mode: 'peer' # See ActionMailer documentation for other possible options
   }
 end
+
+# To use an SMTP connection pool, uncomment the following section:
+#
+# require 'mail/smtp_pool'
+#
+# ActionMailer::Base.add_delivery_method :smtp_pool, Mail::SMTPPool
+#
+# if Rails.env.production?
+#   Rails.application.config.action_mailer.delivery_method = :smtp_pool
+#
+#   ActionMailer::Base.delivery_method = :smtp_pool
+#   ActionMailer::Base.smtp_pool_settings = {
+#     pool: Mail::SMTPPool.create_pool(
+#       pool_size: Gitlab::Runtime.max_threads,
+#       address: "email.server.com",
+#       port: 465,
+#       user_name: "smtp",
+#       password: "123456",
+#       domain: "gitlab.company.com",
+#       authentication: :login,
+#       enable_starttls_auto: true,
+#       openssl_verify_mode: 'peer' # See ActionMailer documentation for other possible options
+#     )
+#   }
+# end