record migration name in marginalia comment

test plan:
 * add migration to your marginalia.yml (or copy over the example
   yml)
 * run a migration
 * check your logs - ensure the migration name is in the
   marginalia comment

Change-Id: I3a9f5163951a40904f1f4fe0d1c5c359dbabb25d
Reviewed-on: https://gerrit.instructure.com/50042
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

config/initializers/active_record.rb

@@ -1256,6 +1256,9 @@ module Migrator
 
   def execute_migration_in_transaction(migration, direct)
     old_in_migration, ActiveRecord::Base.in_migration = ActiveRecord::Base.in_migration, true
+    if defined?(Marginalia)
+      old_migration_name, Marginalia::Comment.migration = Marginalia::Comment.migration, migration.name
+    end
     if down? && !Rails.env.test? && !$confirmed_migrate_down
       require 'highline'
       if HighLine.new.ask("Revert migration #{migration.name} (#{migration.version}) ? [y/N/a] > ") !~ /^([ya])/i
@@ -1267,6 +1270,7 @@ module Migrator
     super
   ensure
     ActiveRecord::Base.in_migration = old_in_migration
+    Marginalia::Comment.migration = old_migration_name if defined?(Marginalia)
   end
 end
 ActiveRecord::Migrator.prepend(Migrator)

config/initializers/marginalia.rb

@@ -9,7 +9,7 @@ if config[:components].present?
   module Marginalia
     module Comment
       class << self
-        attr_accessor :rake_task
+        attr_accessor :migration, :rake_task
 
         def context_id
           RequestContextGenerator.request_id

config/marginalia.yml.example

@@ -11,6 +11,7 @@
 #   - controller
 #   - action
 #   - job_tag
+#   - migration
 #   - rake_task
 #   - hostname
 #   - pid