fix: go vet errors in observability metrics

- Remove unused avg variable in prometheus.go
- Fix import ordering in llm_metrics.go (move net/http to top)

internal/proxy/llm_metrics.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"net/http"
 	"strings"
 	"time"
 
@@ -149,5 +150,4 @@ func (w *ResponseWriterWrapper) Hijack() (net.Conn, *bufio.ReadWriter, error) {
 	return nil, nil, fmt.Errorf("response writer does not implement Hijacker")
 }
 
-// Import http package
-import "net/http"
+