code: func (c *Config) Token(ctx context.Context) (*oauth2.Token, error) {
  	return retrieveToken(ctx, c, url.Values{
  		"grant_type": {"client_credentials"},
  		"scope":      internal.CondVal(strings.Join(c.Scopes, " ")),
  	})
  }
docs: // Token uses client credentials to retreive a token.
  // The HTTP client to use is derived from the context.
  // If nil, http.DefaultClient is used.