code: func NewAltTokenSource(tokenURL, tokenBody string) oauth2.TokenSource {
  	client := oauth2.NewClient(oauth2.NoContext, google.ComputeTokenSource(""))
  	a := &AltTokenSource{
  		oauthClient: client,
  		tokenURL:    tokenURL,
  		tokenBody:   tokenBody,
  		throttle:    flowcontrol.NewTokenBucketRateLimiter(tokenURLQPS, tokenURLBurst),
  	}
  	return oauth2.ReuseTokenSource(nil, a)
  }
docs: // NewAltTokenSource constructs a new alternate token source for generating tokens.