如何从google.oauth2.service_account.Credentials对象的实例中获取访问令牌？

from google.auth.transport.requests import Request

if credentials.expired:
    credentials.refresh(Request())

access_token = credentials.token

from google.auth.transport.requests import Request

# 确保已经完成了身份验证，并获得了Credentials对象的实例
credentials = ...

# 检查访问令牌是否已过期
if credentials.expired:
    credentials.refresh(Request())

# 从Credentials对象中获取访问令牌
access_token = credentials.token