Add logging to process-sync-events service

What does this MR do and why?

With this, we can track;

• how many events we process
• queue depth, event backlog

Log example;

{
  "severity": "INFO",
  // ...
  "class": "Projects::ProcessSyncEventsWorker",
  "jid": "291154d5221998bd27c7441a",
  // ...
  "extra.projects_process_sync_events_worker.total_events": 4,
  "extra.projects_process_sync_events_worker.consumable_events": 4,
  "extra.projects_process_sync_events_worker.processed_events": 4,
  // ...
}

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/346786#note_821346429

Database

Prepare:

INSERT INTO namespaces_sync_events (namespace_id) SELECT id FROM namespaces LIMIT 100000;
VACUUM ANALYZE namespaces_sync_events;

Option 1:

SELECT COUNT(id) FROM namespaces_sync_events;

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/8316/commands/29414

Option 2:

SELECT COALESCE(MAX(id) - MIN(id) + 1, 0) AS upper_bound_count FROM namespaces_sync_events;

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/8316/commands/29415

We choose Option 2.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.