Re-enable org.eclipse.microprofile.metrics.test.MpMetricTest

[jmartisk]

It was excluded after #4954

[jmartisk]

@kenfinnigan No matter how I try, I can't reproduce the failure you mentioned in #4954 (comment) - if I take the current master and un-exclude the test, everything works. Guess I'll just send a PR with un-excluding the test and we'll see how it goes