Prometheus scrapes can fire every few seconds. The open-alerts / open-rules
gauges query Postgres on each read — caching the values for 30s amortises
that to one query per half-minute. Addresses final-review NIT from Plan 02.
- Introduces a package-private TtlCache that wraps a Supplier<Long> and
memoises the last read for a configurable Duration against a Supplier<Instant>
clock.
- Wraps each gauge supplier (alerting_rules_total{enabled|disabled},
alerting_instances_total{state}) in its own TtlCache.
- Adds a test-friendly constructor (package-private) taking explicit
Duration + Supplier<Instant> so AlertingMetricsCachingTest can advance
a fake clock without waiting wall-clock time.
- Adds AlertingMetricsCachingTest covering:
* supplier invoked once per TTL across repeated scrapes
* 29 s elapsed → still cached; 31 s elapsed → re-queried
* gauge value reflects the cached result even after delegate mutates
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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