Too many docvalue_fields reported while there is less

[immon]

Elasticsearch version (bin/elasticsearch --version): 7.9.2

Plugins installed: []

JVM version (java -version): bundled

OS version (uname -a if on a Unix-like system): Elastic Cloud

Description of the problem including expected versus actual behavior:

Search query requesting more docvalue_fields than index configured setting max_docvalue_fields_search throws an error even if requested fields do not exist.

A non-existent field should not contribute to the limit.

Here is minimal reproduction:

PUT docvaluetest
{
  "settings": {
    "max_docvalue_fields_search": 1
  }
}

POST docvaluetest/_doc
{
  "@timestamp": "2020-10-15T11:11:11Z"
}

GET docvaluetest/_search
{
  "query": {
    "match_all": {}
  },
  "docvalue_fields": [
    {
      "field": "@timestamp",
      "format": "date_time"
    },
    {
      "field": "nonexistent1",
      "format": "date_time"
    }
  ]
}

Response:

{
  "error" : {
    "root_cause" : [
      {
        "type" : "illegal_argument_exception",
        "reason" : "Trying to retrieve too many docvalue_fields. Must be less than or equal to: [1] but was [2]. This limit can be set by changing the [index.max_docvalue_fields_search] index level setting."
      }
    ],
    "type" : "search_phase_execution_exception",
    "reason" : "all shards failed",
    "phase" : "query",
    "grouped" : true,
    "failed_shards" : [
      {
        "shard" : 0,
        "index" : "docvaluetest",
        "node" : "YoeZKDjXS5mMBExIOWfgqA",
        "reason" : {
          "type" : "illegal_argument_exception",
          "reason" : "Trying to retrieve too many docvalue_fields. Must be less than or equal to: [1] but was [2]. This limit can be set by changing the [index.max_docvalue_fields_search] index level setting."
        }
      }
    ],
    "caused_by" : {
      "type" : "illegal_argument_exception",
      "reason" : "Trying to retrieve too many docvalue_fields. Must be less than or equal to: [1] but was [2]. This limit can be set by changing the [index.max_docvalue_fields_search] index level setting.",
      "caused_by" : {
        "type" : "illegal_argument_exception",
        "reason" : "Trying to retrieve too many docvalue_fields. Must be less than or equal to: [1] but was [2]. This limit can be set by changing the [index.max_docvalue_fields_search] index level setting."
      }
    }
  },
  "status" : 400
}

Expected behavior would be to first check field existence and only then verify the limits.

[elasticmachine]

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

[cbuescher]

I looked at the way we check the limit in FetchDocValuesContext and think we can check if the field is mapped for that particular index there.
I'll open a PR for discussion if this is the best way to fix this soon.