KAFKA-19947 Add Unit test coverage for KafkaCSVMetricsReporter

[devtrace404]

This PR adds comprehensive unit test coverage for the KafkaCSVMetricsReporter class, which previously had zero test coverage.

Test Results

% ./gradlew :core:test --tests kafka.metrics.KafkaCSVMetricsReporterTest
> Task :core:test

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testCsvFileFormat() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testMultipleStopsAreSafe() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testReporterDoesNotStartWhenDisabled() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testReporterStartsWhenEnabled() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testStopAndRestart() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testManualStart() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testMultipleStartsAreSafe() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testCustomPollingInterval() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testOldDirectoryIsDeleted() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testDirectoryCreation() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testDirectoryCreationWithDefaultPath() PASSED

Gradle Test Run :core:test > Gradle Test Executor 3 > KafkaCSVMetricsReporterTest > testReporterCreationAndMBeanName() PASSED
BUILD SUCCESSFUL in 2m 34s
86 actionable tasks: 86 executed