[Serve][5/n] Add proxy health and request routing delay metrics

[abrarsheikh]

fixes #59218

[gemini-code-assist]

Code Review

This pull request introduces several new metrics to improve the observability of Ray Serve. Specifically, it adds a histogram to measure the routing stats propagation delay from replicas to the controller, a gauge to monitor proxy health status, and another gauge to track when a proxy is in a draining state. The changes are implemented across deployment_state.py, proxy.py, and proxy_state.py, and are accompanied by new tests in test_metrics.py and test_metrics_2.py. The implementation is solid and the new metrics are a great addition. I've identified a minor issue in one of the new tests that could lead to flakiness and have suggested a correction.