[Serve][3/n] Add router queue latency

[abrarsheikh]

fixes #59218

[gemini-code-assist]

Code Review

This pull request introduces a new metric, serve_queue_wait_time_ms, to measure the time requests spend in the router queue. The implementation adds logic to record this latency in RequestRouter and FIFOMixin, and includes a new test case to verify the metric's correctness.

My review focuses on improving code maintainability by addressing duplication and ensuring consistency in metric tagging. I've also identified a bug in the new test case where time units are being compared incorrectly.

Key feedback points:

• Refactor duplicated metric recording logic into a helper method.
• Ensure consistent metric tagging for the new histogram.
• Correct the assertion in the new test to use the correct time unit (milliseconds).