Fix flaky test in MetricRegistryImplTest

[KellyShao]

Fixed flaky test that caused by MetricRegistryImpl in com.google.monitoring.metrics.MetricRegistryImplTest.java.

Existing tests are flaky because the MetricRegistryImpl doesn't clear metrics at the beginning of this class.

The flaky tests are found by running https://github.com/idflakies/iDFlakies

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[KellyShao]

@googlebot I signed it!

[jianglai]

Hi Kelly,

Thanks for the contribution! Sorry I was on leave for a while. I've made a small suggestion. Everything else seems good.

[jianglai]

Can you use @link #unregisterAllMetrics instead?