TypeScript: LabelValues distributes over label-name unions, so labels from get() lose all but one key #861

In 0.16.x, LabelValues is a distributive conditional type:

type LabelValues<T extends string> = T extends NoLabelNameType
	? Partial<Record<string, never>>
	: Partial<Record<T, string | number>>;

Because T is a naked type parameter, TypeScript distributes the conditional over each member of a label-name union. For a metric with labelNames: ['a', 'b'], LabelValues<'a' | 'b'> becomes Partial<Record<'a', string | number>> | Partial<Record<'b', string | number>> rather than a single object with both keys.

Writes still compile, but reading labels back does not, e.g. from get():

import { Counter } from '@prometheus-io/client';

const c = new Counter({ name: 'x_total', help: 'x', labelNames: ['a', 'b'] as const });
c.inc({ a: '1', b: '2' });
const { values } = await c.get();
values[0].labels.a;
// error TS2339: Property 'a' does not exist on type
// 'Partial<Record<"a", string | number>> | Partial<Record<"b", string | number>>'.

This compiled with prom-client 15.1.3, where LabelValues<T> was Partial<Record<T, string | number>>.

Wrapping both sides in a tuple disables distribution and keeps the never (no labels) case working:

type LabelValues<T extends string> = [T] extends [NoLabelNameType]
	? Partial<Record<string, never>>
	: Partial<Record<T, string | number>>;

(As a side effect, the current form also resolves to never rather than Partial<Record<string, never>> when T is never, since a distributive conditional over never yields never.)