Found the way to configure the serviceaccount. There is a telemetry secret that automatically mounted into the telemetry pod as volume ( mixer container have access to it).
@Huy_Le1 happy you were able to sort out your issues. have you looked into the stackdriver extension inside of istio-proxy? Mixer is deprecated, so if you are looking for a lasting way to integrate, you may want to consider it. It would be cool if you could try it out and see if it meets your needs.