Add opt-in Vec TTL via MetricVec and TTLRegistry

[jpinsonneau]

Introduce NewMetricVecWithTTL and per-child lastAccessed tracking so
stale label sets can be omitted from Collect and removed with
CleanupExpired. Store vec children as pointers to satisfy atomic
copying rules.

Keep default NewCounterVec/NewGaugeVec/NewHistogramVec on NewMetricVec
without TTL for backwards compatibility. Opt-in TTLRegistry wraps a
dedicated Registry, exposes New*Vec constructors with a fixed ttl, and
runs CleanupExpired before each Gather.

Relates to #1983

Signed-off-by: Julien Pinsonneau jpinsonn@redhat.com
Made-with: Cursor

[kakkoyun]

Thanks for the PR. This is close to the finish line.

Please see my comments.

[Copilot]

Pull request overview

This PR introduces an opt-in TTL mechanism for *Vec metrics to automatically omit and optionally delete stale label sets, aimed at reducing unbounded growth for high-cardinality / ephemeral labels while preserving backwards compatibility for existing New*Vec constructors.

Changes:

• Add NewMetricVecWithTTL plus CleanupExpired and per-child lastAccessed tracking to support TTL-based expiration.
• Add TTLRegistry, a dedicated registry with New*Vec constructors that enable TTL and run CleanupExpired before each Gather.
• Refactor CounterVec, GaugeVec, and HistogramVec constructors to share internal new*VecWithTTL helpers, and add test coverage for TTL behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
prometheus/vec.go	Adds TTL-enabled MetricVec construction, routes core vec operations through TTL map when enabled, and exposes CleanupExpired.
prometheus/ttl_vec.go	Implements TTL-backed metric storage with per-child lastAccessed, TTL-aware Collect, and cleanupExpired.
prometheus/ttl_registry.go	Introduces TTLRegistry that auto-registers TTL vecs and runs cleanup before Gather.
prometheus/counter.go	Refactors vec creation into newCounterVecWithTTL and wires TTL registry support.
prometheus/gauge.go	Refactors vec creation into newGaugeVecWithTTL and wires TTL registry support.
prometheus/histogram.go	Refactors vec creation into newHistogramVecWithTTL and wires TTL registry support.
prometheus/vec_test.go	Adds TTL behavior tests for Counter/Gauge/Histogram vecs and helper for counting collected metrics.
prometheus/ttl_registry_test.go	Adds tests ensuring NewTTLRegistry validation and that Gather triggers cleanup.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.