metadata setting is not available for Active Directory realms

[jkakavas]

This is probably an unintended consequence of #30241 and thus affects > 7.0 . The setting is only available for LDAP realms as it's defined as

 public static final Setting.AffixSetting<List<String>> ADDITIONAL_META_DATA_SETTING = Setting.affixKeySetting(
            RealmSettings.realmSettingPrefix(LdapRealmSettings.LDAP_TYPE), "metadata",
            key -> Setting.listSetting(key, Collections.emptyList(), Function.identity(), Setting.Property.NodeScope));

Was originally reported in the forums

[elasticmachine]

Pinging @elastic/es-security