docvalue_fetcher for icu_collation_keyword fields fails totally

[nik9000]

Elasticsearch version (bin/elasticsearch --version): master, more?

curl -XDELETE -uelastic:password -HContent-Type:application/json localhost:9200/test
curl -XPUT -uelastic:password -HContent-Type:application/json localhost:9200/test -d'{
  "mappings": {
    "properties": {
      "k": {
        "type": "icu_collation_keyword"
      }
    }
  }
}'
curl -XPOST -uelastic:password -HContent-Type:application/json 'localhost:9200/test/_doc?pretty&refresh' -d'{
  "k": "the quick brown fox"
}'
curl -XPOST -uelastic:password -HContent-Type:application/json localhost:9200/test/_search?pretty -d'{
  "fields": [{"field": "k"}]
}'
# "fields" : {"k": ["the quick brown fox"]}  Great!

curl -XPOST -uelastic:password -HContent-Type:application/json localhost:9200/test/_search?pretty -d'{
  "docvalue_fields": [{"field": "k"}]
}'
# "fields" : {"k" : ["⠜ಁॊ⎢燰Ⴑᢌ噄Țᆖ\"瀑㠀\u0001"]}

[nik9000]

Well, I guess it isn't fair to say they fail totally. I mean, nothing crashes. Its just wrong.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[jtibshirani]

As noted in #53246, we decided not to invest in fixing these issues with docvalue_fields. The recommended way to load these values is through the new fields option instead. As part of our testing efforts around field loading (#70929) we could look into throwing clearer errors for field types that don't work.