feat(stovepipe): report last-green freshness - #572
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Callers gate deployments on a queue's last-known-green commit, so its age is the staleness of the newest thing they are allowed to ship. A queue whose green bookmark stops advancing looks healthy from the pipeline's perspective — nothing is failing — while the answer it serves silently ages, and today nothing makes that visible. Add an observability extension seam alongside the existing ones: a Reporter bound to a single queue by its Factory, whose Report emits one sample of that queue's current state. Reporting is best-effort by contract — Report returns nothing, because an observation that cannot be made must never change what the pipeline records or decides — so implementations emit their own error metrics and callers can defer a report without handling a result. The lastgreen implementation reports that age as a gauge, the first in the repo: the answer is the latest observation rather than a distribution, which the platform/metrics guidance now admits alongside a NamedGauge helper. Dating the bookmarked commit needs a timestamp SourceControl did not expose, so ChangeInfo joins that contract, returning immutable metadata about the commit a URI names rather than about the ref that points at it. The record stage, which owns the bookmark, reports after handling each message.
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The Reporter interface, its Factory, and the lastgreen implementation had one implementation and one caller, and abstracted no technology choice: storage and source control were already extensions, and tally arrives as an injected scope. What remained was domain logic, which moves into the periodicmetrics controller.
Add a periodicmetrics queue stage that emits the age of a queue's last-known-green commit as a gauge. It is the one stage no other stage feeds: the deployment publishes a PeriodicMetrics message on a schedule, because the health it reports degrades while the pipeline is idle and so cannot be observed from pipeline activity alone. A failed observation acks and is counted with the step that failed; only a message violating the payload contract is rejected, so a persistently unresolvable queue cannot fill the dead-letter queue at the publishing rate. record no longer reports, which takes a source-control call off the delivery path of the stage that owns durable validation state.
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Closing - aligned on polling for this internally. |
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Summary
Support periodic observations of queue health via a Reporter interface. This can be called as part of other tasks that we know will alter queue state (wired here into the record step), and also triggered separately on a periodic schedule.
SourceControl will provide one extra method to provide the timestamp of the latest green commit - this will give us a real time gauge of how much time has elapsed since the last commit was marked green.
Since this is a real time observation of current state, and not a collection of observations, a histogram does not make sense for this use case - updated docs to clarify when to use gauge vs. histogram.
Test Plan