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

setting/config.go

@@ -153,6 +153,11 @@ func InitTTSConfig() error {
 		}
 	}
 
+	common.DebugLog = getEnvBool("BYTEDANCE_TTS_DEBUG", false)
+	if common.DebugLog {
+		log.Println("调试日志已启用 BYTEDANCE_TTS_DEBUG")
+	}
+
 	TTSOptions = volcano.Options{
 		APIKey:         apiKey,
 		ResourceID:     resourceId,