[Core][Observability] Add KV cache residency metrics (#27793)

Introduces three new Prometheus histograms for fine-grained observability of KV cache residency behavior:

vllm:kv_block_lifetime_seconds — total lifetime from allocation to free
vllm:kv_block_idle_before_evict_seconds — idle duration before eviction
vllm:kv_block_reuse_gap_seconds — time between consecutive reuses of the same block

These metrics help operators analyze KV cache efficiency, reuse patterns, and eviction timing beyond simple utilization rates.

Implementation uses monotonic timestamps for accuracy, 1% sampling for minimal overhead (~48 bytes/block), and is fully thread-safe with zero runtime cost when disabled.

Two new runtime flags are introduced:

--kv-cache-metrics – enable KV cache residency metrics
--kv-cache-metrics-sample – control sampling ratio (default: 0.01)

Signed-off-by: Shivam <shivamprasad91@gmail.com>

vllm/engine/arg_utils.py

@@ -517,6 +517,10 @@ class EngineArgs:
     collect_detailed_traces: list[DetailedTraceModules] | None = (
         ObservabilityConfig.collect_detailed_traces
     )
+    kv_cache_metrics: bool = ObservabilityConfig.kv_cache_metrics
+    kv_cache_metrics_sample: float = get_field(
+        ObservabilityConfig, "kv_cache_metrics_sample"
+    )
     scheduling_policy: SchedulerPolicy = SchedulerConfig.policy
     scheduler_cls: str | type[object] | None = SchedulerConfig.scheduler_cls
 
@@ -1013,6 +1017,13 @@ class EngineArgs:
             "--collect-detailed-traces",
             **observability_kwargs["collect_detailed_traces"],
         )
+        observability_group.add_argument(
+            "--kv-cache-metrics", **observability_kwargs["kv_cache_metrics"]
+        )
+        observability_group.add_argument(
+            "--kv-cache-metrics-sample",
+            **observability_kwargs["kv_cache_metrics_sample"],
+        )
 
         # Scheduler arguments
         scheduler_kwargs = get_kwargs(SchedulerConfig)
@@ -1698,6 +1709,8 @@ class EngineArgs:
             show_hidden_metrics_for_version=self.show_hidden_metrics_for_version,
             otlp_traces_endpoint=self.otlp_traces_endpoint,
             collect_detailed_traces=self.collect_detailed_traces,
+            kv_cache_metrics=self.kv_cache_metrics,
+            kv_cache_metrics_sample=self.kv_cache_metrics_sample,
         )
 
         # Compilation config overrides