Settings filter not applied to default settings

[javanna]

As part of #29229 we realized that the get index settings API allows to filter the returned settings by providing an expression as part of the url:

curl 'localhost:9200/_settings/index.number*?pretty'
{
  "test" : {
    "settings" : {
      "index" : {
        "number_of_shards" : "5",
        "number_of_replicas" : "1"
      }
    }
  }
}

Such filter is not applied to the default settings though when requested through the include_defaults parameter.

curl 'localhost:9200/_settings/index.number*?pretty&include_defaults=true'
{
  "test" : {
    "settings" : {
      "index" : {
        "number_of_shards" : "5",
        "number_of_replicas" : "1"
      }
    },
    "defaults" : {
      "index" : {
        "max_ngram_diff" : "1",
        "translog" : {
          "generation_threshold_size" : "64mb",
          "flush_threshold_size" : "512mb",
          "sync_interval" : "5s",
          "retention" : {
            "size" : "512mb",
            "age" : "12h"
          },
          "durability" : "REQUEST"
        },
        "auto_expand_replicas" : "false",
        "max_inner_result_window" : "100",
        "mapper" : {
          "dynamic" : "true"
        },
        "requests" : {
          "cache" : {
            "enable" : "true"
          }
        },
        "unassigned" : {
          "node_left" : {
            "delayed_timeout" : "1m"
          }
        },
        "max_terms_count" : "65536",
        "data_path" : "",
        "highlight" : {
          "max_analyzed_offset" : "-1"
        },
        "routing" : {
          "rebalance" : {
            "enable" : "all"
          },
          "allocation" : {
            "enable" : "all",
            "total_shards_per_node" : "-1"
          }
        },
        "search" : {
          "slowlog" : {
            "level" : "TRACE",
            "threshold" : {
              "fetch" : {
                "warn" : "-1",
                "trace" : "-1",
                "debug" : "-1",
                "info" : "-1"
              },
              "query" : {
                "warn" : "-1",
                "trace" : "-1",
                "debug" : "-1",
                "info" : "-1"
              }
            }
          }
        },
        "fielddata" : {
          "cache" : "node"
        },
        "routing_partition_size" : "1",
        "max_docvalue_fields_search" : "100",
        "merge" : {
          "scheduler" : {
            "max_thread_count" : "2",
            "auto_throttle" : "true",
            "max_merge_count" : "7"
          },
          "policy" : {
            "reclaim_deletes_weight" : "2.0",
            "floor_segment" : "2mb",
            "max_merge_at_once_explicit" : "30",
            "max_merge_at_once" : "10",
            "max_merged_segment" : "5gb",
            "expunge_deletes_allowed" : "10.0",
            "segments_per_tier" : "10.0"
          }
        },
        "max_refresh_listeners" : "1000",
        "max_slices_per_scroll" : "1024",
        "shard" : {
          "check_on_startup" : "false"
        },
        "load_fixed_bitset_filters_eagerly" : "true",
        "number_of_routing_shards" : "5",
        "write" : {
          "wait_for_active_shards" : "1"
        },
        "percolator" : {
          "map_unmapped_fields_as_text" : "false",
          "map_unmapped_fields_as_string" : "false"
        },
        "allocation" : {
          "max_retries" : "5"
        },
        "mapping" : {
          "coerce" : "false",
          "nested_fields" : {
            "limit" : "50"
          },
          "depth" : {
            "limit" : "20"
          },
          "ignore_malformed" : "false",
          "total_fields" : {
            "limit" : "1000"
          }
        },
        "refresh_interval" : "1s",
        "indexing" : {
          "slowlog" : {
            "reformat" : "true",
            "threshold" : {
              "index" : {
                "warn" : "-1",
                "trace" : "-1",
                "debug" : "-1",
                "info" : "-1"
              }
            },
            "source" : "1000",
            "level" : "TRACE"
          }
        },
        "compound_format" : "0.1",
        "blocks" : {
          "metadata" : "false",
          "read" : "false",
          "read_only_allow_delete" : "false",
          "read_only" : "false",
          "write" : "false"
        },
        "max_script_fields" : "32",
        "query" : {
          "default_field" : [
            "*"
          ],
          "parse" : {
            "allow_unmapped_fields" : "true"
          }
        },
        "format" : "0",
        "max_result_window" : "10000",
        "sort" : {
          "missing" : [ ],
          "mode" : [ ],
          "field" : [ ],
          "order" : [ ]
        },
        "store" : {
          "stats_refresh_interval" : "10s",
          "type" : "",
          "fs" : {
            "fs_lock" : "native"
          },
          "preload" : [ ]
        },
        "priority" : "1",
        "queries" : {
          "cache" : {
            "everything" : "false",
            "enabled" : "true"
          }
        },
        "ttl" : {
          "disable_purge" : "false"
        },
        "warmer" : {
          "enabled" : "true"
        },
        "codec" : "default",
        "max_rescore_window" : "10000",
        "max_adjacency_matrix_filters" : "100",
        "max_shingle_diff" : "3",
        "gc_deletes" : "60s",
        "optimize_auto_generated_id" : "true",
        "query_string" : {
          "lenient" : "false"
        }
      }
    }
  }
}

I think that default settings should also be filtered, such that the following request yields the following response:

curl 'localhost:9200/_settings/index.auto*?pretty&include_defaults=true'
{
  "test" : {
    "settings" : { },
    "defaults" : {
      "index" : {
        "auto_expand_replicas" : "false"
      }
    }
  }
}

Relates to #29229

[elasticmachine]

Pinging @elastic/es-core-infra

[javanna]

This has been fixed as part of #29229 .