[Metrics] Fix KV cache usage percent metric multiproc (#28792)

The `vllm:kv_cache_usage_perc` Gauge metric is missing `multiprocess_mode="mostrecent"` and ends up returning

```
vllm:kv_cache_usage_perc{engine="0",model_name="Qwen/Qwen3-VL-8B-Instruct",pid="277"} 0.0
vllm:kv_cache_usage_perc{engine="0",model_name="Qwen/Qwen3-VL-8B-Instruct",pid="275"} 0.0
vllm:kv_cache_usage_perc{engine="0",model_name="Qwen/Qwen3-VL-8B-Instruct",pid="273"} 0.6530455880475035
...
```

The deprecated `vllm:gpu_cache_usage_perc` Gauge metric has `multiprocess_mode="mostrecent"`.
Signed-off-by: Jae-Won Chung <jwnchung@umich.edu>

vllm/v1/metrics/loggers.py

@@ -494,6 +494,7 @@ class PrometheusStatLogger(AggregateStatLoggerBase):
         gauge_kv_cache_usage = self._gauge_cls(
             name="vllm:kv_cache_usage_perc",
             documentation="KV-cache usage. 1 means 100 percent usage.",
+            multiprocess_mode="mostrecent",
             labelnames=labelnames,
         )
         self.gauge_kv_cache_usage = make_per_engine(