Google Fit data upload to datasource faild.

[andrew-t902]

Something wrong with Google Fit Rest API.
API request：
POST https://fitness.googleapis.com/fitness/v1/users/me/dataSources/derived:com.google.calories.expended
Response：
{
    "code":503,
    "errors":[
        {
            "domain":"global",
            "message":"The service is currently unavailable.",
            "reason":"backendError"
        }
    ],
    "message":"The service is currently unavailable.",
    "status":"UNAVAILABLE"
}
Please tell me what happen, and when it can be fixed.