Metrics: Measure synthetic source per-request document reconstruction latency

[salvatore-campagna]

Description

In the following months we will see usage of synthetic source increasing as a result of integrations adopting new index modes like tsdb and logs. When using synthetic source we reconstruct the original document using doc values and stored fields instead of just fetching the _source field. This implies larger latencies resulting from having to iterate all fields of a document to reconstruct the source instead of just fetching a single field. We would like to keep track of the time spent to reconstruct documents using synthetic source, measuring the per-request latency to reconstruct all the document sources involved in a specific request.

[elasticsearchmachine]

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

[lkts]

#106732 is merged but we are missing a widget in a dashboard. As part of this work we also want to rename "TSDB" dashboard to "Storage engine" or similar.