chore(xcap): adds exporter to summarise capture as a structured log line

[ashwanthgoli]

What this PR does / why we need it:

1. Adds a log exporter for xcap to summarise observations. Which observations to summarise and export is currently hard-coded, it can be made configurable if needed as the collector api is composable.
2. Exports capture as stats.Result which gets used in query response. Only summary is populated, we can also set dataobj stats but i am not sure if its important as we will using the above summary line for monitoring the new engine.
3. Removes stats collection using logqlmodel/stats for the new engine
4. Move all stats definitions to a central place to avoid circular imports

example:

level=info ts=2025-12-02T14:45:08.324964585Z caller=exporter.go:126 component=query-engine <--REDACTED--> logs_dataset_bucket_get_range=2 logs_dataset_pages_cache_hit=3 logs_dataset_pages_download_duration_ns=47.412432ms logs_dataset_pages_download_requests=1 logs_dataset_pages_scanned=4 logs_dataset_primary_pages_compressed_bytes="33 kB" logs_dataset_primary_pages_downloaded=2 logs_dataset_primary_rows_read=5487 logs_dataset_primary_rows_read_bytes="58 kB" logs_dataset_read_calls=4 logs_dataset_rows_after_pruning=5487 logs_dataset_rows_max=5487 logs_dataset_secondary_pages_compressed_bytes="45 kB" logs_dataset_secondary_pages_downloaded=7 logs_dataset_secondary_rows_read=39 logs_dataset_secondary_rows_read_bytes="40 kB" metastore_bucket_attributes=1 metastore_bucket_get=1 metastore_bucket_get_range=7 metastore_index_objects=1 metastore_primary_pages_compressed_bytes="821 B" metastore_primary_pages_downloaded=9 metastore_resolved_sections=1 metastore_secondary_pages_compressed_bytes="1.5 kB" metastore_secondary_pages_downloaded=13 streams_bucket_get_range=2 streams_primary_pages_compressed_bytes="8 B" streams_primary_pages_downloaded=1 streams_secondary_pages_compressed_bytes="191 B" streams_secondary_pages_downloaded=4

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[ashwanthgoli]

observations names are hardcoded, I plan to move the stat definitions to a central place so we can directly refer to them.

[benclive]

One question on field naming but otherwise LGTM!

[benclive]

Do these keys hold nanoseconds specifically? Prometheus-style metrics usually just use "seconds" as the unit so maybe we want to use that, rather than doing something different?
I believe .String() on a duration already formats it into something like "3.23s" so the nanoseconds piece would get ignored anyway

[ashwanthgoli]

they do hold ns now, i can switch it to secs 👍

[benclive]

Is this queue time accurate? Could we pass it into the ToStatsSummary already and resolve one of the TODOs?

[ashwanthgoli]

i doubt it, in the code i see its being set by the old scheduler. I removed it intentionally to track it propery in a follow-up