Fix flaky MetricsTest.testMetrics

Gradle Test Run :core:test > Gradle Test Executor 1119 > MetricsTest > testMetrics(boolean) > testMetrics with systemRemoteStorageEnabled: false FAILED
    org.opentest4j.AssertionFailedError: Broker metric not recorded correctly for kafka.network:type=RequestMetrics,name=MessageConversionsTimeMs,request=Produce value 0.0 ==> expected: <true> but was: <false>
        at app//org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
        at app//org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
        at app//org.junit.jupiter.api.AssertTrue.failNotTrue(AssertTrue.java:63)
        at app//org.junit.jupiter.api.AssertTrue.assertTrue(AssertTrue.java:36)
        at app//org.junit.jupiter.api.Assertions.assertTrue(Assertions.java:214)
        at app//kafka.api.MetricsTest.verifyBrokerMessageConversionMetrics(MetricsTest.scala:314)
        at app//kafka.api.MetricsTest.testMetrics(MetricsTest.scala:110)