Add explore-cardinality and reduce-metrics-cardinality path packages

[Eve832]

Summary

• Adds two new standalone path packages, explore-cardinality-lj (read the Cardinality tab in the Cost Management and Billing app, weight 1250) and reduce-metrics-cardinality-lj (review Adaptive Metrics recommendations, weight 1260), each with a cover, one interactive milestone, and an end-journey milestone.
• Content is carried verbatim from the reviewed labeling-strategy-lj path (one cross-milestone reference reworded so each package stands alone). Targeting is split from the source manifest: CMAB app URLs for explore, Adaptive Metrics app URLs for reduce.
• These packages are embedded by the upcoming "Design a labeling strategy to control cardinality" Learning Hub journey in grafana/website; that PR depends on this one merging and syncing first (embed URLs /docs/learning-paths/explore-cardinality/ and /docs/learning-paths/reduce-metrics-cardinality/).

Test plan

• Pathfinder CLI validate: both packages and all four milestone sub-packages pass (milestone-level no-targeting WARNs match the repo's path-level-targeting convention)
• Block Editor smoke test of both interactive milestones on a stack with metrics flowing

Made with Cursor

[tacole02]

Thanks for the PR. I smoke-tested both interactive milestones on learn.grafana.net shared in Block Editor (Cardinality tab with metrics flowing, and Adaptive Metrics recommendations). The Show me / Do it flows worked for me. I left a few notes on the diff about noop steps that read like clicks without selectors, and a missing post-section summary on the reduce milestone.

[tacole02]

I smoke-tested this milestone on learn.grafana.net and the flow worked, but a couple of steps tell the learner to scroll to the Highest cardinality metrics table and click a metric row while using noop with no selector. Pathfinder still numbers those as interactive steps even though Show me / Do it cannot help. Prefer markdown for those (or a real highlight if you have a stable row selector).

[tacole02]

Same pattern here on the scroll/review table and "Click a recommendation…" steps: noop with no selector, so they show as interactive steps without automation. Prefer markdown (peer Adaptive Metrics guides do this) unless you can target the table or a row.

[tacole02]

After the section, the next markdown jumps straight into remediation order. A one-sentence "what you learned" summary right after the section (outside it) would close the bookend, then keep the remediation list after that.

[tacole02]

One more packaging note after smoke-testing these in Pathfinder.

Please follow the existing learning-path convention in this repo: one Pathfinder path package with a continuous milestone sequence, not two thin packages that only become one story via a website Hub embed.

Right now Explore and Reduce are separate paths (each basically one interactive milestone + end-journey), so finishing Explore in Pathfinder does not continue into Reduce. That is unlike peer paths here. Those are usually one package with a fuller milestone list in one product surface (for example billing-usage-lj in CMAB soft-suggesting adaptive-metrics-lj, or adaptive-metrics-lj as its own multi-milestone path). Cross-app stories use one continuous path, or two full paths linked with suggests / related_journeys.

Please rework this to match that convention: keep Explore → Reduce as milestones in one Pathfinder path (or reuse/soft-link the existing billing + adaptive paths), rather than shipping two mini-packages for Hub embeds. On the other hand, if you prefer to keep these as separate interactive guides rather than a continuous learning path, that's okay too!