Fix race condition in active user

[SungJin1212]

I have noticed a race condition in active_user.go. (https://github.com/cortexproject/cortex/actions/runs/15336995596/job/43156054957?pr=6763)
This PR fixes it.
The logs are:

WARNING: DATA RACE
Read at 0x00c00411e420 by goroutine 996:
  github.com/cortexproject/cortex/pkg/util.(*ActiveUsers).ActiveUsers()
      /__w/cortex/cortex/pkg/util/active_user.go:98 +0x87
  github.com/cortexproject/cortex/pkg/util.(*ActiveUsersCleanupService).ActiveUsers()
      /__w/cortex/cortex/pkg/util/active_user.go:146 +0x139
  github.com/cortexproject/cortex/pkg/distributor.(*Distributor).updateLabelSetMetrics()
      /__w/cortex/cortex/pkg/distributor/distributor.go:814 +0x104
  github.com/cortexproject/cortex/pkg/distributor.(*Distributor).running()
      /__w/cortex/cortex/pkg/distributor/distributor.go:489 +0x4b1
  github.com/cortexproject/cortex/pkg/distributor.(*Distributor).running-fm()
      <autogenerated>:1 +0x47
  github.com/cortexproject/cortex/pkg/util/services.(*BasicService).main()
      /__w/cortex/cortex/pkg/util/services/basic_service.go:190 +0x3ab
  github.com/cortexproject/cortex/pkg/util/services.(*BasicService).StartAsync.func1.gowrap1()
      /__w/cortex/cortex/pkg/util/services/basic_service.go:119 +0x33

Previous write at 0x00c00411e420 by goroutine 1000:
  runtime.mapassign()
      /usr/local/go/src/internal/runtime/maps/runtime_swiss.go:191 +0x0
  github.com/cortexproject/cortex/pkg/util.(*ActiveUsers).PurgeInactiveUsers()
      /__w/cortex/cortex/pkg/util/active_user.go:80 +0x3f7
  github.com/cortexproject/cortex/pkg/util.(*ActiveUsersCleanupService).iteration()
      /__w/cortex/cortex/pkg/util/active_user.go:138 +0x144
  github.com/cortexproject/cortex/pkg/util.(*ActiveUsersCleanupService).iteration-fm()
      <autogenerated>:1 +0x47
  github.com/cortexproject/cortex/pkg/util.NewActiveUsersCleanupService.NewTimerService.func1()
      /__w/cortex/cortex/pkg/util/services/services.go:33 +0x1d8
  github.com/cortexproject/cortex/pkg/util/services.(*BasicService).main()
      /__w/cortex/cortex/pkg/util/services/basic_service.go:190 +0x3ab
  github.com/cortexproject/cortex/pkg/util/services.(*BasicService).StartAsync.func1.gowrap1()
      /__w/cortex/cortex/pkg/util/services/basic_service.go:119 +0x33

The reason is that go-routines could access m.timestamps simultaneously.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[justinjung04]

Nice! Thanks

[yeya24]

Nice catch 👍