Puma is a fast Ruby webserver with a very small memory footprint. However, if you try to use New Relic with Puma on Heroku, you will see that no data is recorded in New Relic.

To solve that situation, you just need to setup the following Heroku config var:

  heroku config:add NEWRELIC_DISPATCHER=Puma

And voila!, now your New Relic dashboard should be displaying all the metrics about your app.

Update: This is not needed anymore. New Relic now supports Puma out of the box.