Tests: JettyServiceMetricsTest.threadPoolMetrics() is flaky

[swankjesse]

java.lang.AssertionError: 
Expecting:
  <0.6>
to be close to:
  <0.5>
by less than 10% but difference was 19.999999999999996%.
(a difference of exactly 10% being considered valid)
	at misk.web.JettyServiceMetricsTest.threadPoolMetrics(JettyServiceMetricsTest.kt:100)

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