fix(types): stop LabelValues distributing over label-name unions - #862

Summary

Tuple-wrap both sides of the LabelValues check ([T] extends [NoLabelNameType]) so a multi-label get() keeps every declared key.

LabelValues<T> was a naked distributive conditional (T extends never). For a metric with two label names, LabelValues became a union of one-key objects, so const { values } = await counter.get(); values[0].labels.a failed to compile (TS2339) even though both keys were declared. The no-label never case resolves to Partial<Record<string, never>> (the empty-record type #655 intended; the naked form yielded never by distributing over the empty union).

test/typescript.ts reads .method and .status from a two-label Counter and Gauge get(). That access fails tsc on the old type.

Fixes #861.

Decision

Tuple-wrap both sides of the LabelValues check rather than revert to Partial<Record<T, string | number>> (prom-client 15.1.3). The tuple wrap is the form in the issue, and it keeps the no-label empty-record type from #655. Reverting would drop that branch.

Happy to switch if you would rather revert the alias, drop the changelog line, or also cover Histogram/Summary get() reads (same alias).

Test plan

• npm run compile-typescript passes with the wrap.
• Revert only the index.d.ts line; the new get() label reads fail tsc with TS2339 on the one-key union.
• Restore the wrap; tsc is clean again.
• npm run lint and npm run check-prettier pass.