fix(util): Fix flaky TestWorkerPool_TestMetric race condition

[yeya24]

What this PR does

Fixes the flaky TestWorkerPool_TestMetric test in pkg/util.

Root Cause

The test was racy because the first Submit() could hit the select default case (fallback path) if the worker goroutine hadn't started receiving on the channel yet. Since the channel is unbuffered and Submit uses a non-blocking select:

select {
case s.serverWorkerChannel <- f:
default:
    s.fallbackTotal.Inc()
    go f()
}

If the single worker goroutine wasn't ready to receive when the first job was submitted, both jobs would go through the fallback path, incrementing the counter to 2 instead of the expected 1.

Fix

Add a started channel that the first job closes once it begins executing. The test waits on this channel before submitting the second job, ensuring the worker is actually busy.

Verification

Ran the test 50 times with -count=50 and with -race — all pass consistently.

[SungJin1212]

LGTM, can you get the rebase?