runtime: TestReadMetricsCumulative failures in mayMoreStackPreempt mode

[gopherbot]

#!watchflakes
default <- pkg == "runtime:mayMoreStackPreempt" && test == "TestReadMetricsCumulative"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestReadMetricsCumulative
    metrics_test.go:543: /cpu/classes/user:cpu-seconds decreased: 43.052189 < 43.056638
--- FAIL: TestReadMetricsCumulative (0.01s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "runtime:mayMoreStackPreempt" && test == "TestReadMetricsCumulative"

2024-02-16 20:25 gotip-windows-amd64-longtest go@cdd0ddaf runtime:mayMoreStackPreempt.TestReadMetricsCumulative (log)

=== RUN   TestReadMetricsCumulative
    metrics_test.go:543: /cpu/classes/user:cpu-seconds decreased: 43.052189 < 43.056638
--- FAIL: TestReadMetricsCumulative (0.01s)

— watchflakes

[mknyszek]

The fact that this only happened in mayMoreStack mode suggests this is a test-only issue that won't be encountered by users. I'll take a look.