Add item_count metric to field data cache stats API

[peteralfonsi]

Description

Adds an item_count metric to the existing _nodes/stats/indices/fielddata API. Like memory size, it can be aggregated by field by passing a fielddata_fields param listing the desired fields.

Example API response for curl -XGET "localhost:9200/_nodes/stats/indices/fielddata?pretty&fielddata_fields=text0,text2":

"indices" : {
        "fielddata" : {
          "memory_size_in_bytes" : 31184,
          "evictions" : 0,
          "item_count" : 10,
          "fields" : {
            "text0" : {
              "memory_size_in_bytes" : 15580,
              "item_count" : 5
            },
            "text2" : {
              "memory_size_in_bytes" : 15604,
              "item_count" : 5
            }
          }
        }
      }

Related Issues

Resolves #19173

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[peteralfonsi]

Hmm, I had mistakenly renamed a PublicApi method so detect-breaking-change was failing. But now that I've fixed it the job seems to be having some issue with Failed to load eclipse jdt formatter: run 1, run 2

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[sgup432]

This doesn't look right to me. I think we should update FieldMemoryStats itself to have item_count as well along with the existing memory_size_in_bytes ?

And then we can the reuse the existing fields.writeTo and fields.toXContent, and it will automatically spit out the per field item count as well. Also makes more sense logically having item count tracked at FieldMemoryStats level.

[peteralfonsi]

I initially tried to do something like this but there are 2 problems:

1. FieldMemoryStats is used in other places like CompletionStatsCache which don't necessarily need item counts, plus it's PublicApi so we'd have to introduce new ctors with the items as a param + deprecate all the old ones which isn't ideal.
2. The logic of keeping track of memory and item count is identical, besides how it formats the XContent at the end, so I tried having the FieldMemoryStats object itself provide an additional toXContent method which wouldn't format it as a memory. But, now the memory and the item objects don't know they are meant to go together in the higher-level stats object, and we get XContent that looks like this:

So, I figured the best approach was to have the field-level memory and item counts live in separate FieldMemoryStats instances, and let the higher-level stats object handle arranging the counts so they show up properly like

"indices" : {
        "fielddata" : {
          "memory_size_in_bytes" : 31184,
          "evictions" : 0,
          "item_count" : 10,
          "fields" : {
            "text0" : {
              "memory_size_in_bytes" : 15580,
              "item_count" : 5
            }

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[peteralfonsi]

Flaky test: #17678

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