docs: @GetMapping("/sp/metadata")
      public ResponseEntity<String> getFirstServiceProviderMetadata() {
          val saml2Client = builtClients.findClient(SAML2Client.class);
          if (saml2Client != null) {
              return getSaml2ClientServiceProviderMetadataResponseEntity(saml2Client);
          }
          return getNotAcceptableResponseEntity();
      }