fix(types): allow string[] label values

[AysajanE]

Problem

prom-client accepts an array of strings as a label value at runtime — e.g. .inc({ handler: ['/a', '/b'] }), which renders as handler="/a,/b" — but its TypeScript types reject it: the label-values type only allows string | number, so that call fails to type-check even though it works. Reported in #582.

Change

• index.d.ts: widen the label-values type to string | number | string[] so array-valued labels type-check.
• test/typescript.ts: add an array-valued-label usage so tsc proves the type now accepts it.

Checks (Node 22)

• npm run compile-typescript (tsc --project .) and npm run test-unit — both pass.

Fixes #582

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Disclosure: this change was prepared with AI assistance and reviewed by me before submitting.
I ran the check(s) above in a network-isolated container and published a signed, re-runnable
record of that run, verifiable via GitHub artifact attestation:
https://northset-oss.github.io/verification-pilot/. Contributor self-run, not a maintainer verification.

[jdmarshall]

This is a subset of #553 that doesn't really move things forward and there was resistance to landing that one that hasn't really abated.

Just making it work for counter isn't really answering the ticket.