feat(monitoring): chunk fan-out placement panels in default dashboard

[vilenarios]

What

Adds a "Chunk Fan-out Seeding (placement)" row to the provisioned Grafana dashboard (monitoring/grafana/dashboards/default.json), visualizing the metrics from the fault-domain-aware quorum work (#806):

Panel	Shows
Distinct fault domains per POST (avg / p50 / p10)	how many independent operators (IP /24·/48) each chunk lands on. p10 is the seeding-risk tail — low p10 = a real slice of chunks landing on few fault domains
Preferred (tip) shortfall rate by reason	tips_unavailable surfaces the tips-/24 SPOF even when the soft-preferred fallback still succeeds
Distinct-domain target shortfall rate by reason	unmet vs degraded (soft-capped on scarcity)
Chunk broadcast success rate	quorum context

Notes

• Additive only — existing panels are unchanged (verified: 362 insertions, all prior panels intact).
• Panels reference chunk_post_distinct_domains, chunk_post_{preferred,domain}_shortfall_total, and the existing arweave_chunk_broadcast_total. They show "No data" until feat(chunk-post): fault-domain-aware quorum + soft-preferred fallback (B2) #806 merges and the metrics flow — harmless until then.
• Auto-provisioned via monitoring/grafana/provisioning/dashboards (loads on next Grafana start).
• This is the gateway-operator observability surface for the placement work; it is not end-user or bundler-facing (see the boundary RFC for the bundler-side proposal).

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: f1194449-dcf9-4e11-a8ab-bcbdd73787a2

📥 Commits

Reviewing files that changed from the base of the PR and between feefd25 and 04eeeb0.

📒 Files selected for processing (1)

• monitoring/grafana/dashboards/default.json

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📝 Walkthrough

Walkthrough

This PR modifies the Grafana dashboard JSON by reformatting a stat panel's calcs field and adding a new "Chunk Fan-out Seeding (placement)" row panel with four timeseries panels covering fault domains, shortfall rates by reason, and chunk broadcast success rate metrics.

Changes

Grafana Dashboard Update

Layer / File(s)	Summary
New chunk fan-out seeding panels and formatting monitoring/grafana/dashboards/default.json	Adds a new row panel and four timeseries panels (IDs 15–18) with Prometheus quantile queries, legend/tooltip settings, and a percentunit unit for chunk broadcast success rate; also reformats an existing stat panel's calcs array to multi-line.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Suggested reviewers: djwhitt, karlprieb

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: adding chunk fan-out placement panels to the default Grafana dashboard.
Description check	✅ Passed	The description is directly about the same dashboard panels and metrics described in the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/grafana-chunk-fanout-placement-panels

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.76%. Comparing base (96ede32) to head (04eeeb0).
⚠️ Report is 19 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #811      +/-   ##
===========================================
+ Coverage    78.65%   78.76%   +0.10%     
===========================================
  Files          132      132              
  Lines        49980    50093     +113     
  Branches      3763     3780      +17     
===========================================
+ Hits         39312    39454     +142     
+ Misses       10620    10591      -29     
  Partials        48       48              

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[vilenarios]

Superseded by #812 — consolidated into the feature/chunk-fanout integration branch (F + B1 + B2 + dashboard as one coherent PR). All commits from this branch are included there; closing to review as a unit.