Currently there is no way of seeing all settings that are enforced. For instance, defaults are not available and settings from the config/elasticsearch.yml are not available.

Any setting which is different from the default should be returned by these APIs:

GET /{index}/_settings
GET /_cluster/settings

Both should also support the include_defaults query string parameter, which would return all settings, including the defaults.

Also, deleting a setting should reset the setting to the default.

Setting an unknown setting, or a setting that can't be changed should throw an error.

Relates to #5018, #3670, #2738, #2730, #3572, #2628, #3671, #5019, #6309