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".

[SuperQ]

@jdmarshall

So I re-read this section of the license file. This copyright line is not meant to be modified in the LICENSE file. Rather, it's meant that you are to add the section to individual files.

This is what we do in the rest of the Prometheus ecosystem.

You can see in the Prometheus Makefile.common how we apply this to Go code.