Flake in TestPurger_Metrics()

[bboreham]

--- FAIL: TestPurger_Metrics (0.27s)
    purger_test.go:435: 
        	Error Trace:	purger_test.go:435
        	Error:      	Max difference between 259200 and 172800 allowed is 1, but difference was 86400
        	Test:       	TestPurger_Metrics
FAIL

Failed at https://circleci.com/gh/cortexproject/cortex/33030
Passed at https://circleci.com/gh/cortexproject/cortex/33041

[pracucci]

@sandeepsukhani Could you look at it, please?

[sandeepsukhani]

@sandeepsukhani Could you look at it, please?

The problem was that the test was chasing a moving target.
Opened a PR to fix it #2777

[bboreham]

Back again? https://github.com/cortexproject/cortex/runs/1656724172

[sandeepsukhani]

Pushed a possible fix for it #3657.
Most likely it was due to a race between check for expected metric value and updating it.