private String getIdPEndpointUrl() {
        val resolver = SpringExpressionLanguageValueResolver.getInstance();
        return resolver.resolve(configurationContext.getCasProperties().getServer().getPrefix().concat("/idp"));
    }
Write metadata.

@param metadata          the metadata
@param registeredService registered service
@return the string
@throws Throwable the throwable
support/cas-server-support-saml-idp-core/src/main/java/org/apereo/cas/support/saml/idp/metadata/generator/BaseSamlIdPMetadataGenerator.java