feat(cag): memory standing-context adapter, warmup semaphore, cache-aware routing

Issue #44: code-register memory adapter with standing-context resource; fix
dispatcher method-level Auth.Capability override (was always using verb-derived
capability, ignoring per-method Auth field).

Issue #45: per-backend bounded in-memory queue (VLLM_MAX_QUEUE_DEPTH=20) +
concurrency semaphore (VLLM_MAX_NUM_SEQS=4); warmup requests (X-Poimen-Warmup:
true) sleep WARMUP_DELAY_MS before queue entry; /metrics endpoint wired.

Issue #46: rendezvous-hash backend selection on X-Poimen-Cache-Key; round-robin
fallback; MarkFailed excludes unhealthy backends 30s; ModifyResponse captures
vllm-num-cached-tokens + emits structured log + increments counter.

Closes #44 Closes #45 Closes #46

internal/observability/metrics.go

@@ -36,6 +36,10 @@ type Metrics struct {
 	itlMs map[string][]int64 // samples for histogram
 	// Token counts
 	tokenCount map[string]int64
+
+	// CAG metrics
+	warmupRequestsDelayed int64
+	prefixCacheHitTokens  int64
 }
 
 // NewMetrics creates a new Metrics instance.
@@ -280,4 +284,46 @@ func (m *Metrics) Reset() {
 	m.ttftMs = make(map[string][]int64)
 	m.itlMs = make(map[string][]int64)
 	m.tokenCount = make(map[string]int64)
+	m.warmupRequestsDelayed = 0
+	m.prefixCacheHitTokens = 0
+}
+
+// IncrWarmupDelayed increments warmup_requests_delayed_total.
+func (m *Metrics) IncrWarmupDelayed() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.warmupRequestsDelayed++
+}
+
+// GetWarmupDelayed returns warmup_requests_delayed_total.
+func (m *Metrics) GetWarmupDelayed() int64 {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	return m.warmupRequestsDelayed
+}
+
+// IncrPrefixCacheHitTokens increments vllm_prefix_cache_hit_tokens_total by n.
+func (m *Metrics) IncrPrefixCacheHitTokens(n int64) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.prefixCacheHitTokens += n
+}
+
+// GetPrefixCacheHitTokens returns vllm_prefix_cache_hit_tokens_total.
+func (m *Metrics) GetPrefixCacheHitTokens() int64 {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	return m.prefixCacheHitTokens
+}
+
+// globalMetrics is the singleton Metrics instance for gateway-wide telemetry.
+var (
+	globalMetrics     *Metrics
+	globalMetricsOnce sync.Once
+)
+
+// Global returns the singleton Metrics instance, initialising on first call.
+func Global() *Metrics {
+	globalMetricsOnce.Do(func() { globalMetrics = NewMetrics() })
+	return globalMetrics
 }