Follow-up improvements to flattened object fields.

[jtibshirani]

This meta-issue tracks improvements to flattened fields. The initial version of the feature was designed + tracked in #33003.

Immediate improvements:

• Add support for additional query types like wildcard, regex, and fuzzy.
• Clean-up and add benchmarks to the metricbeat rally track.
• Add a way to retrieve all keys in the JSON object

Requires feedback/ research:

• Explore support for numeric values. As an example, users may want to perform true range queries on numeric fields within the object. Support for a fully numeric flattened field #61550
• Explore adding support for highlighting, since with large JSON blobs it can be difficult to tell which key-value pairs matched the query.
• Introduce a way for certain JSON keys to be 'promoted' into individual fields. One approach we're considering is to extend copy_to to work on entire objects, so that the same JSON blob could be added both as a 'flattened object' field, and also as normal object with explicit subfield definitions.

[elasticmachine]

Pinging @elastic/es-search

[mrkamel]

+1 for supporting a numeric flattened type

[user718]

+1 on list of all available keys from flattened object

[TinaHeiligers]

+1 for supporting a numeric flattened type for ui_metric mappings in the Telemetry cluster indices

[yshyshkin]

+1 for supporting a numeric flattened type