fix(metrics): restore kv cache hit-rate name

Author: nvyutwu

lib/bindings/python/src/dynamo/prometheus_names.py

@@ -271,6 +271,8 @@ class kvstats:
     TOTAL_BLOCKS = "total_blocks"
     # GPU cache usage as a percentage (0.0-1.0)
     GPU_CACHE_USAGE_PERCENT = "gpu_cache_usage_percent"
+    # Prefix cache hit rate (0.0-1.0), portable across vLLM / SGLang / TRT-LLM
+    KV_CACHE_HIT_RATE = "kv_cache_hit_rate"
 
 
 class labels:

lib/runtime/src/metrics/prometheus_names.rs

@@ -823,6 +823,9 @@ pub mod kvstats {
 
     /// GPU cache usage as a percentage (0.0-1.0)
     pub const GPU_CACHE_USAGE_PERCENT: &str = "gpu_cache_usage_percent";
+
+    /// Prefix cache hit rate (0.0-1.0), portable across vLLM / SGLang / TRT-LLM
+    pub const KV_CACHE_HIT_RATE: &str = "kv_cache_hit_rate";
 }
 
 // Model information metrics