Docs: Add 2.1 Metric details page, Assets tab, metric column lineage, and hierarchical lineage map

[kiran1287]

Closes #581

Summary

Documents the 2.1 Metrics and lineage changes in the v2.1.x-SNAPSHOT tree.

• Metrics page (data-governance/metrics.mdx)
  • Overview rewritten as bulleted lists.
  • "Creating Metrics Using the UI" is now plain numbered steps with new screenshots.
  • New Metric Details page section: tabs, Overview panels, Metric Hierarchy, Definition, Related Metrics.
  • New section for linking, browsing, and removing assets on the Assets tab.
• Column-level lineage (data-lineage/column.mdx)
  • New "View Column-Level Lineage" and "Generate Column-Level Lineage Automatically" sections.
  • Manual edit rewritten as numbered steps with screenshots.
  • New Metric Column Lineage section.
• Explore lineage (data-lineage/explore.mdx)
  • New Hierarchical Lineage Map section.
  • Lineage Layers rewritten for the 2.1 Layers panel (Lineage Layer and Level). The old Column and Observability layer sections are removed.

Source PRs: OpenMetadata#32335, #34168, #33168, #29204. Verified against the OpenMetadata source pinned in openmetadata-collate (42c07f4).

Not documented

• Dimensions and Measures widgets (OpenMetadata#30944): not in the pinned source. OpenMetadata#32335 replaced that work with the Metric Hierarchy and Definition cards.

Review notes

• The Metric Column Lineage steps are written from source and need a check in the UI.
• Steps for adding a table in lineage edit mode and removing a link are based on the Collate manual lineage page and screenshots.
• The Observability layer is not described because it is not in the 2.1 Layers panel in source. Confirm in the UI.
• The manual-edit screenshot is from the Collate UI and shows the older lineage toolbar.
• Same change is in docs-collate PR #791.

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Review Report

Content type: Documentation
Overall verdict: NEEDS WORK
Reason: Critical=0, Major=1, Minor=2
Reviewed revision: 91da488
Findings: Critical=0 Major=1 Minor=2

Issues Found

#	Guideline	Severity	Original text	Suggested change
1	Consistent terminology: use one name for each action (Category 2)	Major	"2. Click Add Metric." (metrics.mdx, creation steps, about line 72; "4. Click Create to save the metric." in the same list). "2. Click Edit (the pencil icon) in the lineage toolbar." (column.mdx, about line 62)	The same procedures use "Select" for other UI actions, such as "Select Govern > Metrics" and "Select Edit (the pencil icon) in the lineage toolbar" under "Link a Column to a Metric". Use one verb for UI actions in all new steps. "Select" is the dominant choice.
2	Meaning is unambiguous (Category 4)	Minor	"Spaces, underscores, and dots are not supported (for example, customerRetentionRate)." (metrics.mdx, Name field, about line 80)	The example sits next to the unsupported characters, so it reads as an unsupported value. Move it: "Enter a unique identifier in camelCase, starting with a lowercase letter (for example, customerRetentionRate). Spaces, underscores, and dots are not supported."
3	Clear and concise: remove repetition (Category 2)	Minor	"Use the Impact Analysis view for a table of upstream and downstream assets. It lists the upstream and downstream assets in a table." (explore.mdx, "Edit Mode and Impact Analysis", last bullet)	Keep one sentence: "Use the Impact Analysis view to see upstream and downstream assets in a table."

_{Inputs are identical to an earlier review on this PR (same policy, diff, discussion, and model), so that report is reposted without a new model call.}

_{View job · claude-sonnet-5-5 (high) · reused (no model call)}