def exchange_credential(
      self,
      auth_scheme: AuthScheme,
      auth_credential: Optional[AuthCredential] = None,
  ) -> AuthCredential:
    """Exchanges the provided authentication credential for a usable token/credential.

    Args:
        auth_scheme: The security scheme.
        auth_credential: The authentication credential.

    Returns:
        An updated AuthCredential object containing the fetched credential.
        For simple schemes like API key, it may return the original credential
        if no exchange is needed.

    Raises:
        NotImplementedError: If the method is not implemented by a subclass.
    """
    raise NotImplementedError("Subclasses must implement exchange_credential.")
Exchanges the provided authentication credential for a usable token/credential.

    Args:
        auth_scheme: The security scheme.
        auth_credential: The authentication credential.

    Returns:
        An updated AuthCredential object containing the fetched credential.
        For simple schemes like API key, it may return the original credential
        if no exchange is needed.

    Raises:
        NotImplementedError: If the method is not implemented by a subclass.
    
src/google/adk/tools/openapi_tool/auth/credential_exchangers/base_credential_exchanger.py