Remove mirror_scheduling_tracker and ProjectImporScheduleWorker's needs_own_queue tag

What does this MR do and why?

For #340630 (closed)

In !81249 (merged), we introduced a counter to track the scheduling jobs of ProjectImporScheduleWorker. This counter acts as a replacement for ProjectImportScheduleWorker.queue_size in UpdateAllMirrorsWorker. The counter usage was hidden behind mirror_scheduling_tracker. The flag as rolled out successfully a while ago.

This MR cleans up the tag and corresponding code paths. Besides, it also removes needs_own_queue tag from ProjectImportScheduleWorker. This change set that worker's queue to default, instead of project_import_schedule. The shard handling jobs from this worker stays the same - still catchall.

Screenshots or screen recordings

N/A

How to set up and validate locally

Validate the functionality of UpdateAllMirrorsWorker

• Start one Rails console session (Console A):

loop do
  puts "current_scheduling: #{Gitlab::Mirror.current_scheduling}"
  puts "queue_size: #{ProjectImportScheduleWorker.queue_size}"
  sleep 0.5
end

• Start another Rails console session (Console B). Run the following command to start an UpdateAllMirrorsWorker:

UpdateAllMirrorsWorker.new.perform_async

• Console A prints out the following lines, indicates that Gitlab::Mirror.current_scheduling and actual queue size are very close, some seconds apart. Looking at the logs, UpdateAllMirrorsWorker job is blocked until the number drops to 0.

job_tracker: 19
queue_size: 19
job_tracker: 12
queue_size: 10
job_tracker: 4
queue_size: 3
job_tracker: 2
queue_size: 0
job_tracker: 0
queue_size: 0
...

• Look at Sidekiq admin dashboard and logs to see UpdateAllMirrorsWorker is rescheduled

Validate the queue of ProjectImportScheduleWorker

• Step 1: Set the routing_rules configuration in config/gitlab.yml to following:

        - ["tags=needs_own_queue", null]
        - ["worker_name=AuthorizedProjectUpdate::UserRefreshFromReplicaWorker,AuthorizedProjectUpdate::UserRefreshWithLowUrgencyWorker", "quarantine"] # move this to the quarantine shard
        - ["worker_name=RepositoryImportWorker", "imports"] # imports
        - ["resource_boundary=cpu&urgency=high", "urgent_cpu_bound"] # urgent-cpu-bound
        - ["resource_boundary=memory", "memory_bound"] # memory-bound
        - ["feature_category=global_search&urgency=throttled", "elasticsearch"] # elasticsearch
        - ["resource_boundary!=cpu&urgency=high", "urgent_other"] # urgent-other
        - ["resource_boundary=cpu&urgency=default,low", "low_urgency_cpu_bound"] # low-urgency-cpu-bound
        - ["feature_category=database&urgency=throttled", "database_throttled"] # database-throttled
        - ["feature_category=gitaly&urgency=throttled", "gitaly_throttled"] # gitaly-throttled
        - ["*", "default"] # catchall on k8s

• Step 2: check the queue of ProjectImportScheduleWorker

[1] pry(main)> ProjectImportScheduleWorker.queue
=> "default"

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.