protected void mergeAuthenticationAttributes(final Map<String, List<Object>> authenticationAttributes,
                                                 final AttributeMerger merger, final Authentication authn) {
        authenticationAttributes.putAll(CoreAuthenticationUtils.mergeAttributes(authenticationAttributes, authn.getAttributes(), merger));
        LOGGER.debug("Finalized authentication attributes [{}] for inclusion in this authentication result", authenticationAttributes);
    }
Principal id is and must be enforced to be the same for all authentications.
Based on that restriction, it's safe to grab the first principal id in the chain
when composing the authentication chain for the caller.
core/cas-server-core-authentication-api/src/main/java/org/apereo/cas/authentication/DefaultAuthenticationResultBuilder.java