Backfill sharding key on resource_label_events

What does this MR do and why?

• Backfill resource_label_events table with correct sharding key based on the issuable (issue, merge_request, epic)
• Fix invalid records (records that have both epic_id and issue_id set) during backfill (source of the error fixed in !196618 (merged))
• Add sync namespace_id index Adding in Add resource_label_events.namespace_id sync index (!197983 - merged) so we can start the review on this one before the weekend. We don't need the index for the backfill as we are not introducing the FK yet.
• Add NOT VALID FK on namespace_id column Not adding Fk for safety on the move feature as copying existing records could potentially cause errors if existing records are not backfilled first. Adding and validating FK constraint after backfill is finalized in %18.4

DB review

Clean epic_id records that also had issue_id set

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/41544/commands/127575

WITH relation AS (
  SELECT 
    "resource_label_events".* 
  FROM 
    "resource_label_events" 
  WHERE 
    "resource_label_events"."id" BETWEEN 419550600 
    AND 419559999 
    AND "resource_label_events"."id" >= 419550600 
    AND "resource_label_events"."id" < 419550800 
  LIMIT 
    200
), filtered_relation AS (
  SELECT 
    * 
  FROM 
    relation 
  WHERE 
    (
      num_nonnulls(epic_id, issue_id) = 2
    ) 
  LIMIT 
    200
) 
UPDATE 
  "resource_label_events" 
SET 
  "issue_id" = NULL 
FROM 
  "filtered_relation" 
WHERE 
  "resource_label_events"."id" = "filtered_relation"."id"

Set namespace_id for issue related records

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/41544/commands/127578

WITH relation AS (
  SELECT 
    "resource_label_events".* 
  FROM 
    "resource_label_events" 
  WHERE 
    "resource_label_events"."id" BETWEEN 419550600 
    AND 419559999 
    AND "resource_label_events"."id" >= 419550600 
    AND "resource_label_events"."id" < 419550800 
  LIMIT 
    200
), filtered_relation AS (
  SELECT 
    * 
  FROM 
    relation 
  WHERE 
    issue_id IS NOT NULL 
  LIMIT 
    200
) 
UPDATE 
  "resource_label_events" 
SET 
  "namespace_id" = "issues"."namespace_id" 
FROM 
  "filtered_relation" 
  INNER JOIN "issues" ON "filtered_relation"."issue_id" = "issues"."id" 
WHERE 
  "resource_label_events"."id" = "filtered_relation"."id"

Set namespace_id for merge request related records

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/41544/commands/127579

WITH relation AS (
  SELECT 
    "resource_label_events".* 
  FROM 
    "resource_label_events" 
  WHERE 
    "resource_label_events"."id" BETWEEN 419550600 
    AND 419559999 
    AND "resource_label_events"."id" >= 419550600 
    AND "resource_label_events"."id" < 419550800 
  LIMIT 
    200
), filtered_relation AS (
  SELECT 
    * 
  FROM 
    relation 
  WHERE 
    merge_request_id IS NOT NULL 
  LIMIT 
    200
) 
UPDATE 
  "resource_label_events" 
SET 
  "namespace_id" = "projects"."project_namespace_id" 
FROM 
  filtered_relation 
  INNER JOIN merge_requests ON filtered_relation.merge_request_id = merge_requests.id 
  INNER JOIN projects ON projects.id = merge_requests.target_project_id 
WHERE 
  "resource_label_events"."id" = "filtered_relation"."id"

Set namespace_id for epic related records

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/41544/commands/127580

WITH relation AS (
  SELECT 
    "resource_label_events".* 
  FROM 
    "resource_label_events" 
  WHERE 
    "resource_label_events"."id" BETWEEN 419550600 
    AND 419559999 
    AND "resource_label_events"."id" >= 419550600 
    AND "resource_label_events"."id" < 419550800 
  LIMIT 
    200
), filtered_relation AS (
  SELECT 
    * 
  FROM 
    relation 
  WHERE 
    epic_id IS NOT NULL 
  LIMIT 
    200
) 
UPDATE 
  "resource_label_events" 
SET 
  "namespace_id" = "epics"."group_id" 
FROM 
  filtered_relation 
  INNER JOIN epics ON filtered_relation.epic_id = epics.id 
WHERE 
  "resource_label_events"."id" = "filtered_relation"."id"

MR acceptance checklist

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Related to #514594 (closed)