VSD - Implement the rest of the metrics

Add the following metrics to the previously implemented counter worker.

• merge_requests
  • iterate: Namespace.where('traversal_ids[1] = ?', group.id).where(type: 'Project')
  • count: merge_requests
  • column: iid
• projects
  • iterate: Namespace.where('traversal_ids[1] = ?', group.id).where(type: 'Project')
  • count: namespaces
  • column: id
• groups
  • iterate: Namespace.where('traversal_ids[1] = ?', group.id).where(type: 'Group')
  • count: namespaces
  • column: id
• pipelines
  • iterate: Namespace.where('traversal_ids[1] = ?', group.id).where(type: 'Project')
  • count: ci_pipelines
  • column: iid
• members
  • iterate: Namespace.where('traversal_ids[1] = ?', group.id).where(type: 'Group')
  • count: members
  • column: user_id (use distinct_each_batch for counting, see: #394705 (comment 1322288009))

It seems like we can have two strategies:

• uses each_batch_count
• uses distinct_each_batch (only for the members count)