Inconsistent naming of "filter" vs. "filters" when defining analyzer on an index vs. specifying its parameters inline

[fsoikin]

In the snippet below, index analyzertest1 has an analyzer defined with filters, and analyzertest2 - with filter. The latter works, the former doesn't (see also #16451).

When I specify filters inline, the situation is reversed: filters works, filter doesn't.

post analyzertest1
  {
    "settings": {
        "analysis": {
            "analyzer": {
                "my_analyzer": {
                    "tokenizer": "standard",
                    "filters": ["lowercase"]
                }
            }
        }
    }
  }

post analyzertest1/_analyze 
{
   "analyzer": "my_analyzer", "text": "ABCD"
}
// Result: "ABCD"

post analyzertest2
  {
    "settings": {
        "analysis": {
            "analyzer": {
                "my_analyzer": {
                    "tokenizer": "standard",
                    "filter": ["lowercase", "stop"]
                }
            }
        }
    }
  }

post analyzertest2/_analyze 
{
   "analyzer": "my_analyzer", "text": "ABCD"
}
// Result: "abcd"

post _analyze 
{
   "tokenizer": "standard",
   "filter": ["lowercase"],
   "text": "ABCD" 
}
// Result: error "unknown parameter [filter]"

post _analyze 
{
   "tokenizer": "standard",
   "filters": ["lowercase"],
   "text": "ABCD" 
}
// Result: "abcd"

[fsoikin]

/cc: @markharwood

[javanna]

I think there are two problems here:

1. we don't validate (yet) analyzer settings, so non valid ones are accepted and stored in the cluster state. Same as Defining analyzer on index doesn't complain about unknown parameters #16451 .

2. analyze api and create index api are inconsistent, same as analyze api accepts "filters" while create index relies on "filter" #15189.

I will close this issue as duplicate then as we already have specific issues for each of the corresponding problems. Thanks for opening this though ;)