Truncate SAML Response in trace log (elastic#51237) (elastic#51283)

When not truncated, a long SAML response XML document can fill max
line length and mask the actual exception message that the trace
statement is meant to inform about.
The same XML Document is also printed in full on trace level in
SamlRequestHandler#parseSamlMessage() so there is no loss of
information

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/saml/SamlAuthenticator.java

@@ -68,7 +68,8 @@ SamlAttributes authenticate(SamlToken token) {
             try {
                 return authenticateResponse(root, token.getAllowedSamlRequestIds());
             } catch (ElasticsearchSecurityException e) {
-                logger.trace("Rejecting SAML response {} because {}", SamlUtils.toString(root), e.getMessage());
+                logger.trace("Rejecting SAML response [{}...] because {}", Strings.cleanTruncate(SamlUtils.toString(root), 512),
+                    e.getMessage());
                 throw e;
             }
         } else {