docs: update metrics design doc to use new vllm:kv_cache_usage_perc#30041

markmc merged 2 commits intovllm-project:mainvllm-project/vllm:mainfrom

haitwang-cloud:rename-gpu-cache-metric-deprecationhaitwang-cloud/vllm:rename-gpu-cache-metric-deprecationCopy head branch name to clipboard