fix: kv cache panel in grafana.json

Author: Fangping Shi

examples/online_serving/prometheus_grafana/grafana.json

@@ -852,7 +852,7 @@
             "uid": "${DS_PROMETHEUS}"
           },
           "editorMode": "code",
-          "expr": "vllm:gpu_cache_usage_perc{model_name=\"$model_name\"}",
+          "expr": "vllm:kv_cache_usage_perc{model_name=\"$model_name\"}",
           "instant": false,
           "legendFormat": "GPU Cache Usage",
           "range": true,