chore: Synchronize common files from prometheus/prometheus

[prombot]

Propagating changes from prometheus/prometheus default branch.

Source can be found here.

To check out this branch locally and push changes back:

git remote add prombot https://github.com/prombot/prometheus_client_js.git
git fetch prombot repo_sync
git checkout -b repo_sync prombot/repo_sync

[SuperQ]

We do need to add a NOTICE file to update the copyright assignment to "The Prometheus Authors".

[krajorama]

cc @zbjornson

[SuperQ]

Thanks @jdmarshall.