Fix flaky MetricsTest by using polling assertions

Replace fixed Thread.sleep(600ms) + immediate assertions with
assertEventually() polling pattern. The previous approach was
susceptible to flakiness in CI environments where 600ms wasn't
always sufficient for metrics to be reported.

The fix uses the existing assertEventually utility which polls
every 100ms with a 5-second timeout, matching the pattern already
used in io.temporal.client.functional.MetricsTest.

Author: mfateev