Switch from `prom-client` to `@prometheus-io/client`.

prom-client was moved under the official prometheus org umbrella. The package for it has just been released.

The release 0.16.0 should bring no (major) breaking changes - mostly performance improvements. There are few things that need to be checked, though: https://github.com/prometheus/client_js/blob/HEAD/CHANGELOG.md#0160---2026-08-24

Current users: @envelop/prometheus (peer + dev) @graphql-yoga/plugin-prometheus (peer + dev)

Things to be aware of:

1. Engine requirement is now node 22+ - yoga/envelop libs are still on 18+
2. Using global registry by default might result in inconsistencies between registries if not overridden correctly via package manager etc.
3. prom-client will soon be deprecated (https://github.com/prometheus/client_js/issues/750)

Happy to patch what's necessary, but I assume none of this can happen before next Yoga/Envelop version bump that raises minimum engine version.