feat(observability): add a metrics-pipeline smoke test

[JSONbored]

Summary

Phase C of the observability audit — final item, closing the gap that let two dangling metric
references (fixed in #5302) go undetected for weeks: nothing validated that a dashboard's metric
references actually exist and flow through the pipeline.

scripts/smoke-observability-metrics.mjs mirrors the existing smoke-observability-traces.mjs's
shape: push a synthetic OTLP metric through the collector, confirm it surfaces on its Prometheus
exporter (:8889), and separately sanity-check the app's own /metrics HELP/TYPE shape (an
independent check — the collector path couldn't mask a break in the app's own in-process registry).
Wired as npm run test:smoke:observability:metrics, documented in the self-hosting troubleshooting
doc's new "Grafana metrics/dashboards show no data" section (sibling to the existing traces section).

Test plan

• npm run typecheck — clean
• npm run test:ci — full local gate green
• New test in selfhost-observability-config.test.ts covering the script's content and the
  package.json script wiring, mirroring the existing trace-smoke-script test

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	gittensory-ui	0f48732	Commit Preview URL Branch Preview URL	Jul 12 2026, 02:08 PM

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.40%. Comparing base (cf58317) to head (0f48732).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5342   +/-   ##
=======================================
  Coverage   94.40%   94.40%           
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  Files         550      550           
  Lines       44116    44116           
  Branches    14631    14631           
=======================================
  Hits        41649    41649           
  Misses       1792     1792           
  Partials      675      675           

Flag	Coverage Δ
shard-1	44.01% <ø> (ø)
shard-2	34.64% <ø> (ø)
shard-3	31.54% <ø> (ø)
shard-4	31.42% <ø> (ø)
shard-5	33.15% <ø> (ø)
shard-6	43.35% <ø> (ø)

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed · 🟪 Reviewing}