TSDB followups

[martijnvg]

Not completed tasks from #74660 that we may want to followup at some point in the future.

• [ ] Move IndexRequest#autoGeneratId? It's a bit spook where it is but I don't like it any other place.
• [ ] Improve error messages in _update_by_query when modifying the dimensions or @timestamp
• [ ] On translog replay and recovery and replicas we regenerate the _id and assert that it matches the _id from the primary. Should we? Probably. Let's make sure.
• [ ] Treating data stream/index as a dimension
• [ ] Support text field labels in downsampling
• [ ] Histograms support in downsampling
• [ ] SQL/ES|QL support for downsampling
• [x] Optimization of merge policies (#87684)
• [ ] Default the setting's value to all of the keyword dimensions
• [ ] Support shard splitting on time_series indices
• [ ] Make an object or interface for _id's values. Right now it's a String that we encode with Uid.encodeId. That was reasonable. Maybe it still is. But it feels complex and for tsdb who's _id is always some bytes. And encoding it also wastes a byte about 1/128 of the time. It's a common prefix byte so this is probably not really an issue. But still. This is a big change but it'd make ES easier to read. Probably wouldn't really improve the storage though.
• [ ] Figure out how to specify tsdb settings in component templates. For example index.routing_path can be specified in a composable index template if data stream template' index_mode is set to time_series. But if this setting is specified in a component template then it is required to also set the index.mode index setting. This feels backwards. @martijnvg
• [x] In order to retrieve the routing values (defined in index.routin_path), the source needs to be parsed on coordinating node. However in the case that an ingest pipeline is executed this, then the source of document will be parsed for the second time. Ideally the routing values should be extracted when ingest is performed. Similar to how the @timestamp field is already retrieved from a document during pipeline execution.
• [ ] In order to determine the backing index a document should be to, a timestamp is parsed into Instant. The format being used is: strict_date_optional_time_nanos||strict_date_optional_time||epoch_millis. This to allow regular data format, data nanos date format and epoch since mills defined as string. We can optimise the data parsing if we know the exact format being used. For example if on data stream there is parameter that indices that exact data format we can optimise parsing by either using strict_date_optional_time_nanos, strict_date_optional_time or epoch_millis.

[elasticsearchmachine]

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

[mbudge]

Please fix this

https://github.com/elastic/elasticsearch/issues/104839

[elasticsearchmachine]

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