Add TTFT & ITL Metrics for LLM Inference (#28)

Time-to-First-Token (TTFT) and Inter-Token Latency (ITL) metrics for LLM inference observability

Metrics: llm_ttft_seconds, llm_itl_seconds, llm_tokens_total

Closes #31 #32 #33

---------

Co-authored-by: poimen <[email protected]>
Reviewed-on: #28

internal/serviceadapter/types.go

@@ -1,6 +1,7 @@
 package serviceadapter
 
 import (
+	"net/http"
 	"time"
 )
 
@@ -49,11 +50,14 @@ type Status struct {
 }
 
 // ServiceAdapter is a gateway service adapter.
+// When Handler is set, the dispatcher routes directly to the internal handler
+// instead of reverse-proxying to Spec.Upstream.URL.
 type ServiceAdapter struct {
-	Name      string // namespace/name
-	Namespace string
+	Name        string // namespace/name
+	Namespace   string
 	ServiceName string
-	Spec      Spec
-	Status    Status
-	CreatedAt time.Time
+	Spec        Spec
+	Status      Status
+	CreatedAt   time.Time
+	Handler     http.Handler `json:"-" yaml:"-"` // internal handler (skip serialization)
 }