Flake in TestPurger_Metrics()

[bboreham]

https://github.com/cortexproject/cortex/pull/4511/checks?check_run_id=3768593384

--- FAIL: TestPurger_Metrics (2.40s)
    purger_test.go:438: expected 0, got 172800
FAIL
FAIL	github.com/cortexproject/cortex/pkg/chunk/purger	40.455s

Note previous #2776, #2840

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[aknuds1]

This also flaked for #4602, see build.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.