[BUG] Opensearch Index Pattern Refresh Field List cannot handle nested fields

[jhenley-icr-team]

Describe the bug

When attempting to look at data that may contain multiple fields within the json, ex
"config": {
"buildNum": 9007199254740991,
"defaultIndex": "d3d7af60-4c81-11e8-b3d7-01146121b73d"
},
The Opensearch refresh field list fails to find any of the nested fields other than the first. Making it impossible to filter on those values.

To Reproduce
Steps to reproduce the behavior:

1. Go to index patterns page and create an index pattern pointing to .dashboards
2. After creating the index pattern attempt to view a config record
3. Notice multiple config.#### variables are marked "No Cached Mapping exists"
4. Refresh field list for the index pattern
5. Go back to discover and notice the multiple config.#### variables are marked "No Cached Mapping exists" issue is still present

Expected behavior
The refresh index pattern mapping should be able to handle multiple fields being nested, to allow users to see their types and filter on them.

OpenSearch Version
2.18.0

Dashboards Version
2.18.0

Plugins
None

Screenshots