[Router] Update qps when no requests

[Shaoting-Feng]

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FIX #173 (link existing issues this PR will resolve)

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[Shaoting-Feng]

To test the QPS, we can start the router locally. We observe that after the default time window, even if no new requests arrive, the issue is now resolved—the QPS updates correctly.

To test num_prefill_requests, we can start the pods. However, I don’t believe the issue described in Issue #173 is present. I think the error in Issue #173 occurred because the router was started locally, and the request didn’t receive a response. As a result, the num_prefill_requests wouldn’t change.