zgcGenerationalTest is flaky on CI

[shakuzen]

See, for example, https://app.circleci.com/pipelines/github/micrometer-metrics/micrometer/6982/workflows/66800576-a887-4030-b36a-5068d498950b/jobs/35044. JvmGcMetricsTest > gcTimingIsCorrectForPauseCycleCollectors() fails fairly often when running the zgcGenerationalTest task during the CI build. Unfortunately I think it's a matter of whether a GC happens during the test code running that is collecting numbers to make assertions about the metrics having the right numbers for GCs. I don't have a great idea how to improve this.

> Task :micrometer-core:zgcGenerationalTest

JvmGcMetricsTest > gcTimingIsCorrectForPauseCycleCollectors() FAILED
    org.opentest4j.AssertionFailedError: 
    expected: 11L
     but was: 10L
        at app//io.micrometer.core.instrument.binder.jvm.JvmGcMetricsTest.lambda$checkPhaseCount$3(JvmGcMetricsTest.java:236)