Allow specify dynamic templates in bulk request

[dnhatn]

This change allows users to specify dynamic templates in a bulk request.

PUT myindex
{
  "mappings": {
    "dynamic_templates": [{
      "time_histograms": {
        "mapping": {
          "type": "histogram",
          "meta": {
            "unit": "s"
          }
        }
      }
    }]
  }
}

// Tells ES to use the dynamic template named `time_histograms` for field `response_times`
// in the index request
POST myindex/_bulk
{ "index": { "dynamic_templates": { "response_times": "time_histograms" } } }
{ "@timestamp": "2020-08-12", "response_times": { "values": [1, 10], "counts": [5, 1] }}

Closes #61939

[elasticmachine]

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

[javanna]

I left some questions. The approach looks good code-wise overall, my comments are mostly about the API.

Another one: does the matched "type" become the content of the dynamic_type variable? Or should it even? I am wondering myself if this should be considered and promoted as a type. To me it is more of an arbitrary name that defined dynamic templates can match against. Should we rather call it match_mapping_hint?

[dnhatn]

@javanna Thank you for the review. I tweaked the API using "match_mapping_hint" and I found it's simpler and less confusing. I have updated the PR. Can you take another look?

[axw]

@dnhatn apologies if this is obvious: will this be immediately usable through an Ingest script processor (and if so how?), or would that come later?

[dnhatn]

can users use one of these types as hint in their bulk requests, in which case they won't have to do define anything on the dynamic template side of things? Or should hints always be arbitrary and explicitly defined in the dynamic template?

That's a good question. Let's discuss it after this iteration.

[jimczi]

The change looks great @dnhatn ! I left some comments but I think the feature is ready.

[javanna]

I left some more comments, nothing major though. I like how simple the implementation, thanks for iterating on it. Would it make sense to add docs as part of this PR or do you plan to do it as a followup?

[javanna]

I left some minor comments, LGTM otherwise. Thanks for all the iterations.

[dnhatn]

Thanks everyone :).