feat: add debug log control and optimize error response and cors log

- add DebugLog global variable controlled by BYTEDANCE_TTS_DEBUG env
- enable debug logs for cors, tts upstream, response parsing etc when debug mode is on
- update tts error response to use standardized json error format
- add dashboard and metrics path to cache-control exempt list

middleware/security.go

@@ -12,7 +12,7 @@ func SecurityHeaders(next http.Handler) http.Handler {
 		w.Header().Set("X-XSS-Protection", "1; mode=block")
 		w.Header().Set("Referrer-Policy", "strict-origin-when-cross-origin")
 
-		if strings.HasPrefix(r.URL.Path, "/v1/") || r.URL.Path == "/health" {
+		if strings.HasPrefix(r.URL.Path, "/v1/") || r.URL.Path == "/health" || r.URL.Path == "/dashboard" || r.URL.Path == "/metrics" {
 			w.Header().Set("Cache-Control", "no-store")
 		}