[BUG] The comments did not match the codes in MultiTermsAggregationBuilder

[Zhikai-VM]

Describe the bug

In MultiTermsAggregationBuilder.java, the comments about multiple terms aggregation did not match codes:

 GET test_000001/_search
 {
      "size": 0,
      "aggs": {
        "hot": {
          "multi_terms": {
            "terms": [{
              "field": "region"
            },{
              "field": "host"
            }],
            "order": {"max-cpu": "desc"} <====
          },
          "aggs": {
            "max-cpu": { "max": { "field": "cpu" } }
          }
        }
      }
 }

The comment shows we can set up one order. But from the codes, it also supports an array like:
"order": [ {"max-cpu": "desc"} , {"max-memory": "desc"} ]

Here are the codes to parser this field:

PARSER.declareObjectArray(MultiTermsAggregationBuilder::order, (p, c) -> InternalOrder.Parser.parseOrderParam(p), ORDER_FIELD);

I also go through the codes and had some local tests. I think multiple orders do work. If so, could you please update the comment and the official document about this?

Related component

Search:Aggregations

To Reproduce

Compare the comments and codes.

Expected behavior

If multiple orders are supported, we should update the comments and the official document.

Additional Details

No response

[dblock]

Thanks @Zhikai-VM, looks like a real issue. Do you think you could help with the updates? Make PRs?

[Zhikai-VM]

Thanks @Zhikai-VM, looks like a real issue. Do you think you could help with the updates? Make PRs?

Hi @dblock, yes, I could update the comment in a PR, but could you please help to double-confirm the case?

{
     "size": 0,
     "aggs": {
       "hot": {
         "multi_terms": {
           "terms": [{
             "field": "region"
           },{
             "field": "host"
           }],
           "order": [{"max-cpu": "desc"},  {"max-memory": "desc"}]  <==== multiple orders
         },
         "aggs": {
           "max-cpu": { "max": { "field": "cpu" } },
           "max-memory": { "max": { "field": "memory" } }
         }
       }
     }
}

Meanwhile, I think we also need to update the official document in: https://opensearch.org/docs/latest/aggregations/bucket/multi-terms/

[dblock]

I am not very familiar with this functionality, but I think we can work through this!

1. Find an existing YAML REST test that has 1 order.
2. See if there's a test with 2 orders, if not add one with expected change in results.
3. Now that (1) and (2) are confirmed with tests we can be sure, and can document/update with cofidence.

[Zhikai-VM]

I am not very familiar with this functionality, but I think we can work through this!

1. Find an existing YAML REST test that has 1 order.
2. See if there's a test with 2 orders, if not add one with expected change in results.
3. Now that (1) and (2) are confirmed with tests we can be sure, and can document/update with cofidence.

Hi @dblock ,
I created a PR to add unit test to verify the multiple orders in multiple terms aggregation, could you please help to review it:
#13400 ?