Fix flaky MetricsTest by using polling assertions #2045

Job	Run time
Unit test with docker service [JDK8]	11m 59s
Unit test with cloud	22s
Code format	46s
Unit test with in-memory test service [Edge]	7m 18s
Build native test server / Build native test server (ubuntu-latest, linux, amd64, true)	6m 16s
Build native test server / Build native test server (macos-latest, macOS, arm64)	5m 5s
Build native test server / Build native test server (ubuntu-24.04-arm, linux, arm64)	6m 14s
Build native test server / Build native test server (ubuntu-latest, linux, amd64, false)	5m 23s
Build native test server / Build native test server (macos-15-intel, macOS, amd64)	6m 24s
Build native test server / Build native test server (windows-latest, windows, amd64)	6m 11s
	55m 58s

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