Merge branch 'pb-track-background-job-executions' into 'master'

changelogs/unreleased/pb-track-background-job-executions.yml

...0128172149_create_background_migration_tracking_tables.rb

db/schema_migrations/20210128172149

db/structure.sql

...d_migration/copy_column_using_background_migration_job.rb

lib/gitlab/database/background_migration/batched_job.rb

...gitlab/database/background_migration/batched_migration.rb

...atabase/background_migration/batched_migration_wrapper.rb

...ase/background_migration/primary_key_batching_strategy.rb

lib/gitlab/database/background_migration/scheduler.rb

lib/gitlab/database/migration_helpers.rb

...ries/gitlab/database/background_migration/batched_jobs.rb

...itlab/database/background_migration/batched_migrations.rb

...ration/copy_column_using_background_migration_job_spec.rb

.../gitlab/database/background_migration/batched_job_spec.rb

...b/database/background_migration/batched_migration_spec.rb

...se/background_migration/batched_migration_wrapper_spec.rb

...ackground_migration/primary_key_batching_strategy_spec.rb

...ib/gitlab/database/background_migration/scheduler_spec.rb

spec/lib/gitlab/database/migration_helpers_spec.rb