[Meta] Improve the lookup by mget use case

[not-napoleon]

Description

Tracking work on doing large mgets for key-value style lookups

• https://github.com/elastic/elasticsearch/issues/85232
• https://github.com/elastic/elasticsearch/issues/85069
• Mget should support fields option for BWC, like search does.
• Per-request concurrency would help ensure a single mget request can take advantage of all of the CPU and I/O of data nodes. Currently, this requires users to either split their mget requests into multiple ones that they run concurrently, or to split their shards only to make Elasticsearch do more stuff in parallel.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[dnhatn]

I've opened https://github.com/elastic/elasticsearch/pull/85345 for https://github.com/elastic/elasticsearch/issues/85069 (Multi-GET should reuse wrapped readers)

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)