Feature Request: Remove restriction on copy_to to non-child fields

[veqryn]

I have data the is coming in like this, which I am unable to change:
{"tags":["foo=bar", "foo=quux", "blah=blarg"]}

You can see that there are keys (foo and blah) and values (bar, quux, and blarg).

I would like to be able to run aggregation queries against this in order to give me all the keys, with a sub-aggregation for all their values. (ie: how many times does "foo" key occur, and how many times does each of its bar and quux values occur?)

I could normally do this with a nested type, so I tried to make one:

{
  "mappings": {
    "thing": {
      "properties": {
        "tags": {
          "type": "keyword",
          "similarity": "boolean",
          "copy_to": ["ntags.key", "ntags.val"]
        },
        "ntags": {
          "type": "nested",
          "dynamic": false,
          "properties": {
            "key": {
              "type": "keyword",
              "similarity": "boolean",
              "normalizer": "key_normalizer"
            },
            "val": {
              "type": "keyword",
              "similarity": "boolean",
              "normalizer": "value_normalizer"
            }
          }
        }
      }
    }
  }
}

The key_normalizer just takes the string "foo=bar" and turns it into "foo". The value_normalizer turns it into "bar".

Turns out this is not allowed:
Illegal combination of [copy_to] and [nested] mappings: [copy_to] may only copy data to the current nested document or any of its parents, however one [copy_to] directive is trying to copy data from nested object [null] to [ntags]

So I also tried adding ntags as a multi-field underneath tags, but go this error, not that it would have helped since the copy_to probably doesn't work on sub-multi-fields anyway:
Type [nested] cannot be used in multi field

I can't change the format my data comes in as.

So... Is there something else I can do, either at index time or query time?
And if not, can the restriction on copy_to be removed?

(Some additional context here: https://discuss.elastic.co/t/how-to-sub-aggregate-an-array-that-contains-another-field/152155 )

[elasticmachine]

Pinging @elastic/es-search-aggs

[elasticsearchmachine]

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

[javanna]

This has been open for quite a while, and hasn't had a lot of interest. For now I'm going to close this as something we aren't planning on implementing. We can re-open it later if needed.