observability: detect missing scrape series

[ajac-zero]

Summary

• make the collector-down alert fire when PostgreSQL collection reports failure or the entire Agentbay metric series disappears
• document scrape disappearance semantics
• cover the absent() expression in Helm static and E2E rendering tests

Production evidence

The expression evaluates empty while the chart-managed scraper is healthy. Mimir currently reports the updated rule as inactive with health=ok and no evaluation error.

Verification

• Helm lint and static rendering
• 14 non-cluster Helm tests passed
• previous isolated live-cluster Helm suite remains green; the latest rerun hit a pre-test Testcontainers startup failure
• live Mimir expression and ruler validation
• diff whitespace validation

[dispatch-reviewer]

Review: observability: detect missing scrape series

CI Summary

All required checks passed (✅ CI, ✅ Dependency Review). CodeQL is still in-progress but is not a blocking gate for this PR.

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Code Correctness

deploy/helm/agentbay/templates/observability.yaml — PromQL expression

-  expr: max(agentbay_observability_collector_up) == 0
+  expr: (max(agentbay_observability_collector_up) == 0) or absent(agentbay_observability_collector_up)

The logic is correct and well-reasoned:

• The original expression only fires when the metric exists and reports 0 (collector explicitly down). It stays silent when the entire scrape series disappears — a blind spot on pod crash or misconfiguration.
• Adding or absent(agentbay_observability_collector_up) covers that gap. absent() returns a scalar 1 when the time series is completely absent from the TSDB, so the or correctly unifies both failure modes under the same alert.
• Parenthesising the left side ((max(...) == 0)) before the or is good practice; PromQL operator precedence would evaluate this the same way without parens, but explicit grouping prevents any future ambiguity and is more readable.
• The 5-minute for duration applies to both branches equally, which is the right behaviour — transient scrape gaps won't fire spuriously.

deploy/helm/agentbay/README.md

Documentation wording is accurate and concise. No issues.

deploy/helm/agentbay/tests/static-render.sh and test/e2e/helm-chart.test.ts

Both test additions correctly assert the new absent(...) expression appears in rendered output. The shell grep -q and the TypeScript regex /absent\(agentbay_observability_collector_up\)/ both match the exact string produced by the template. Test coverage is proportional to the change — nothing was left untested.

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No Regressions Identified

• The change is purely additive to the alert expression; the existing max(...) == 0 arm is preserved unchanged.
• No other rules, templates, or application logic are affected.
• All 14 non-cluster Helm tests and typecheck/unit/audit CI steps passed cleanly.

LGTM — ready to merge.