Fix logging: add response_bytes to middleware, cache_key to handler

- Middleware now tracks and logs bytes written via response_bytes
- Handler logs cache_key for cache hit debugging
- Changed "served encrypted image" to "image served" (only URL is encrypted)

internal/middleware/middleware.go

@@ -58,11 +58,12 @@ func ipFromHostPort(hp string) string {
 
 type loggingResponseWriter struct {
 	http.ResponseWriter
-	statusCode int
+	statusCode   int
+	bytesWritten int64
 }
 
 func newLoggingResponseWriter(w http.ResponseWriter) *loggingResponseWriter {
-	return &loggingResponseWriter{w, http.StatusOK}
+	return &loggingResponseWriter{ResponseWriter: w, statusCode: http.StatusOK}
 }
 
 func (lrw *loggingResponseWriter) WriteHeader(code int) {
@@ -70,6 +71,13 @@ func (lrw *loggingResponseWriter) WriteHeader(code int) {
 	lrw.ResponseWriter.WriteHeader(code)
 }
 
+func (lrw *loggingResponseWriter) Write(b []byte) (int, error) {
+	n, err := lrw.ResponseWriter.Write(b)
+	lrw.bytesWritten += int64(n)
+
+	return n, err
+}
+
 // Logging returns a logging middleware.
 func (s *Middleware) Logging() func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
@@ -90,6 +98,7 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 					"proto", r.Proto,
 					"remoteIP", ipFromHostPort(r.RemoteAddr),
 					"status", lrw.statusCode,
+					"response_bytes", lrw.bytesWritten,
 					"latency_ms", latency.Milliseconds(),
 				)
 			}()