[WIP] increase mapping field meta char limit

[seanstory]

Note: I never put this through review, and am unsure if increasing the default here would cause any significant or nuanced issues.

I did this work with the intent to evaluate how much increased field-level meta (for descriptions) would help an LLM in picking which indices should be searched, given a natural language prompt. Unfortunately, it became clear quite quickly that this approach wouldn't scale. Even just sending 20 index mappings with no metadata was too much for the EIS LLM to process in a single message context.

Instead, I'm taking an approach outlined in elastic/kibana#232437, to attempt to improve our index selection via top-level descriptions and/or a flat listing of field names.

We may revisit this topic, but it's probably important first to explore making index metadata searchable, before worrying about expanding how much index metadata we can even set.

to use this PR

1. ./gradlew localDistro
2. set the elasticsearch.yml with:

   xpack:
     inference:
       elastic:
         url: "http://localhost:8443"
     license:
       self_generated:
         type: trial
     security:
       enabled: true
   

3. set elasticsearch passwords with:

   bin/elasticsearch-setup-passwords interactive <<EOF
   Y
   changeme
   changeme
   changeme
   changeme
   changeme
   changeme
   changeme
   changeme
   changeme
   changeme
   changeme
   changeme
   EOF
   

4. set the index settings on an index template:

   curl -X PUT -u elastic:changeme \
     -H "Content-Type: application/json" \
     "http://localhost:9200/_index_template/default-meta-limit" \
     -d '{
       "index_patterns": ["<your indices here>"],
       "priority": 5000,
       "template": {
         "settings": {
           "index.mapping.meta.length_limit": 800
         }
       }
     }'
   

5. Run evaluations/experiments

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🔍 Preview links for changed docs

• docs/reference/elasticsearch/mapping-reference/mapping-field-meta.md

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