Add responsive throttling for placeholder user records reassignment

What does this MR do and why?

This adds responsive throttling to placeholder user records reassignment in order to ease pressure on prod DB tables.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• https://gitlab.com/gitlab-org/gitlab/-/issues/493977

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. pull and check out into branch
2. restart the gdk: gdk restart
3. in the gdk console ensure :batched_migrations_health_status_autovacuum and ::batched_migrations_health_status_wal feature flags are enabled
4. patch the autovacuum_active_on_table health status indicator to dummy a stop signal:

diff --git a/lib/gitlab/database/health_status/indicators/autovacuum_active_on_table.rb b/lib/gitlab/database/health_status/indicators/autovacuum_active_on_table.rb
index 6bf2bbf0c707..ef1838a2b445 100644
--- a/lib/gitlab/database/health_status/indicators/autovacuum_active_on_table.rb
+++ b/lib/gitlab/database/health_status/indicators/autovacuum_active_on_table.rb
@@ -10,6 +10,7 @@ def initialize(context)
           end
 
           def evaluate
+            return Signals::Stop.new(self.class, reason: "simulating stop locally")
             return Signals::NotAvailable.new(self.class, reason: 'indicator disabled') unless enabled?
 
             autovacuum_active_on = active_autovacuums_for(tables)

5. open tail the importer log in a terminal window: tail -f log/importer.log | jq
6. ensure you have a valid import_source_user and user to reassign collaborations to and from
7. in the console follow the steps to reassign and accept reassignment:

source_user = Import::SourceUser.find(ID) # Source User 

user = User.find(ID) # Reassign to this user 

::Import::SourceUsers::ReassignService.new(source_user, user, current_user: User.first).execute 

::Import::SourceUsers::AcceptReassignmentService.new(source_user, current_user: user, reassignment_token: source_user.reassignment_token).execute

8. observe in the warning logged in the importer log:

{
  "feature_category": "importers",
  "severity": "WARN",
  "time": "2024-11-29T12:22:28.784Z",
  "correlation_id": "08950dbd3701c5c35fae8fde38fae81d",
  "message": "Database unhealthy. Rescheduling reassignment",
  "source_user_id": 490
}

9. remove the patch on the autovacuum_active_on_table indicator and apply it to the write_ahead_log indicator:

--git a/lib/gitlab/database/health_status/indicators/write_ahead_log.rb b/lib/gitlab/database/health_status/indicators/write_ahead_log.rb
index 253af7c9f0de..a1e6fa1aad17 100644
--- a/lib/gitlab/database/health_status/indicators/write_ahead_log.rb
+++ b/lib/gitlab/database/health_status/indicators/write_ahead_log.rb
@@ -36,6 +36,7 @@ def initialize(context)
           end
 
           def evaluate
+            return Signals::Stop.new(self.class, reason: "Simulating for test")
             return Signals::NotAvailable.new(self.class, reason: 'indicator disabled') unless enabled?
 
             unless pending_wal_count

10. reassign and accept reassignment for another import_source_user
11. observe in the warning logged in the importer log:

{
  "feature_category": "importers",
  "severity": "WARN",
  "time": "2024-11-29T12:22:28.784Z",
  "correlation_id": "08950dbd3701c5c35fae8fde38fae81d",
  "message": "Database unhealthy. Rescheduling reassignment",
  "source_user_id": 491
}

Related to #493977