diagnostics/metrics: collapse GetOrCreate* onto generic helpers

[yperbasis]

The five GetOrCreate{Histogram,Counter,Gauge,SummaryExt,GaugeVec} methods repeated the same ~32-line double-checked-locking block; each copy differed only in the constructor and type assertion. They collapse onto getOrCreate[T prometheus.Metric] / getOrCreateVec[T prometheus.Collector], and each method is now a 3-liner.

The GaugeVec copy is where clone mutation had gone wrong: its type guard compared reflect.TypeFor[*prometheus.GaugeVec]() to itself — always false, i.e. dead (and unreachable while namedMetricVec.metric was concretely typed *prometheus.GaugeVec). The generic widens the field to prometheus.Collector and performs a real assertion with the same error shape as the scalar family (TestSetGetOrCreateGaugeVecWrongType). registerMetricVec is removed: its re-check-and-insert was a duplicate of the inline double-checked insert.

First commit adds characterization tests (+127 lines) pinning existing GetOrCreate* behavior — same-instance returns, wrong-type errors, describe-once semantics — green against the old code before the refactor.

Update (third commit): getOrCreate/getOrCreateVec were themselves clones differing only in which registry (m/a vs vecs/av) they touched. They now delegate to a single getOrCreateIn over a shared namedEntry[M] entry type (namedMetric/namedMetricVec become type aliases), with the wrappers widening T through an adapter closure so the store stays compile-time safe. The double-checked locking becomes one critical section: the fast path took the same mutex anyway, and the create funcs are pure parseMetric + constructor calls, so creating under the lock eliminates the create-twice-drop-loser race. The unlock is deferred because prometheus constructors can panic on inputs reachable through the public API (a quantile/le const label, a negative summary window); a non-deferred unlock would strand the set's mutex. Also drops the dead scalar namedMetric.isAux — never assigned, so the ListMetricNames filter on it was unreachable.

Net −169 lines in set.go. Verified: package tests incl. -race, make lint clean, full go build ./....

Part of a dedup series; siblings #22165, #22166, #22167.

[Copilot]

Pull request overview

This PR refactors diagnostics/metrics.Set by collapsing the duplicated GetOrCreate* implementations into two generic helpers (getOrCreate for scalar metrics and getOrCreateVec for collector/vec metrics), while also correcting GetOrCreateGaugeVec’s wrong-type handling (now a real type assertion instead of an ineffective reflect comparison). It also adds characterization tests to lock in existing behaviors (same-instance reuse, wrong-type errors, invalid-name errors, and Describe semantics).

Changes:

• Replace repeated double-checked-locking blocks in GetOrCreate{Histogram,Counter,Gauge,SummaryExt} with getOrCreate[T prometheus.Metric].
• Rework GetOrCreateGaugeVec to use getOrCreateVec[T prometheus.Collector], widen stored vec type to prometheus.Collector, and perform a real type assertion for wrong-type errors.
• Add tests to pin GetOrCreate* behavior, including concurrent access.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
diagnostics/metrics/set.go	Deduplicates GetOrCreate logic via generics; fixes GaugeVec wrong-type assertion by storing vecs as prometheus.Collector.
diagnostics/metrics/set_test.go	Adds characterization tests for same-instance reuse, wrong-type/invalid-name errors, Describe-once behavior, and concurrency.

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