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

[jdmarshall]

This is just wrong...

[jdmarshall]

Never send a robot to do a human's job. This is all wrong. But probably suggests a change upstream