Identify unnecessary retrievals of _source

[tylerperk]

When synthetic source is enabled, it can be costly to retrieve the _source field, both in latency (we have to reconstruct the source) and S3 calls (in the case of Searchable Snapshots or Serverless data). It's possible there are Elastic components like Kibana Discover that are fetching _source when maybe they actually need specific fields, and so there might be a more efficient way to accomplish their goal.

Let's identify who is fetching _source where and why, so we can discuss alternatives. How is TBD but we could log accesses to source or implement specific telemetry or maybe use APM.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[jdcryans]

We discussed this as a team, there's no need for this research right now and it hasn't come up in discussions since it was filed. Closing.