Fully shard bulk_import_trackers

BulkImportTrackers (# rows: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/41965/commands/128839)

Milestone 18.2 (required stop)

• backfill sharding key -> !197155 (merged)
• backfill sharding key 100% complete

Milestone 18.3

• finalize BBM -> !197974 (merged)
• verify 0 records with NULL sharding key -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/41750/commands/128144

💥 The work to fully shard bulk_import_trackers needs to be restarted (see !188587 (comment 2651635058))

• remove existing triggers on sharding key columns -> !201035 (merged)

Milestone 18.5 (required stop)

• create a new trigger that sets exactly one column of the multi-column sharding key leaving the others as NULL -> !202296 (merged)
  • add a NOT NULL (NOT VALID) constraint on exactly one column of the sharding key for new && updated records
  • backfill sharding key using the newest set of triggers
• sharding key backfill completes 100%
• verify 0 records with NULL sharding key -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/43850/commands/134486

Milestone 18.6

• prepare to fully shard table -> !188587 (merged)
  • finalize the sharding key backfill
  • prepare ASYNC check constraint validation on sharding key
• verify NOT NULL (NOT VALID) -> NOT NULL on .com following the ASYNC constraint validation
• Fully shard table
  • validate SYNC NOT NULL sharding key
  • update db/docs/*.yml with sharding key
• verify the NOT NULL constraint check is set on the table's sharding key on .com