Fix flakiness in JettyClientMetricsWithObservationTest.activeTimer()

[izeye]

I saw the following failure locally:

JettyClientMetricsWithObservationTest > activeTimer(WireMockRuntimeInfo) FAILED
    org.opentest4j.AssertionFailedError:
    expected: 1
     but was: 0
        at java.base@17.0.14/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at java.base@17.0.14/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
        at java.base@17.0.14/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
        at java.base@17.0.14/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
        at app//io.micrometer.jetty12.client.JettyClientMetricsWithObservationTest.activeTimer(JettyClientMetricsWithObservationTest.java:56)

It could be reproducible with @RepeatedTest(100) locally, and the Develocity Build Scan also flags it as a flaky one.

Looking into its code, there seems to be a race condition, but I couldn't find a clean way to fix it.

This PR attempts to fix its flakiness even though it's still relying on timing.