Remove ci_observe_pipelines_finished feature flag

What does this MR do and why?

Remove ci_observe_pipelines_finished feature flag

The ci_observe_pipelines_finished flag introduced in !244288 (merged) was a short-lived gitlab_com_derisk flag used to de-risk the rollout of finished-pipeline metrics on GitLab.com. It reached 100% of gprd on 2026-07-10 and has been stable since, so per the rollout issue #605233 (closed) the flag is removed and the metrics now emit unconditionally.

Also documents pipelines_finished_total and gitlab_ci_pipeline_time_to_finished_seconds in the Prometheus metrics reference, deferred from the introducing MR while the flag was temporary.

Changelog: added

Also baselines a LabKit logging offense in .labkit_logging_todo.yml: UserExperienceSli.observed emits a deprecated start_time field internally, but start_time: is its required keyword arg, so the field can't be fixed at the call site — baselining is the documented escape hatch.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.