index slowlog settings override other indices settings

[jkakavas]

Elasticsearch version : Tested with 6.6.1 and master

Description of the problem including expected versus actual behavior:
Slowlog settings are supposed to be per index. However, setting the slowlog settings for an index, override the respective settings in all other indices, even when these other settings have been previously configured.

Steps to reproduce:

1. Create index test_1 and set index.search.slowlog.level: warn and index.search.slowlog.threshold.query.trace to 0.

curl -X PUT  -H 'Content-Type: application/json' 'http://localhost:9200/test_1/_settings' -d '{"settings": {"index.search.slowlog.level": "warn", "index.search.slowlog.threshold.query.trace": 0}}'

Get the settings to verify it is set correctly

curl -uelastic:password -X GET 'http://localhost:9200/test_1/_settings'

should return

...
"test_1" : {
    "settings" : {
      "index" : {
        "search" : {
          "slowlog" : {
            "level" : "warn",
            "threshold" : {
              "query" : {
                "trace" : "0"
              }
            }
          }
        },
...

2. Make a few searches

curl -X GET 'http://localhost:9200/test_1/_search'

and verify nothing is printed in the *.index_search_slowlog.log

3. Create index test_2 and set index.search.slowlog.level: trace and index.search.slowlog.threshold.query.trace to 0.

curl -X PUT  -H 'Content-Type: application/json' 'http://localhost:9200/test_2/_settings' -d '{"settings": {"index.search.slowlog.level": "warn", "index.search.slowlog.threshold.query.trace": 0}}'

Get the settings to verify it is set correctly

curl -uelastic:password -X GET 'http://localhost:9200/test_2/_settings'

should return

...
"test_2" : {
    "settings" : {
      "index" : {
        "search" : {
          "slowlog" : {
            "level" : "trace",
            "threshold" : {
              "query" : {
                "trace" : "0"
              }
            }
          }
        },
...

Verify also that the settings for test_1 have not been changed ( at least that's reported ):

curl -uelastic:password -X GET 'http://localhost:9200/test_1/_settings'

should still return

...
"test_1" : {
    "settings" : {
      "index" : {
        "search" : {
          "slowlog" : {
            "level" : "warn",
            "threshold" : {
              "query" : {
                "trace" : "0"
              }
            }
          }
        },
...

4. Make a search in test_1 again

curl -X GET 'http://localhost:9200/test_1/_search'

Observe log lines such as

[2019-05-23T13:27:38,633][TRACE][i.s.s.query              ] [node-0] [test_1][0] took[218.5micros], took_millis[0], total_hits[0 hits], types[], stats[], search_type[QUERY_THEN_FETCH], total_shards[1], source[{}], id[],

in the *.index_search_slowlog.log

[elasticmachine]

Pinging @elastic/es-core-infra