New Relic can report on application server resource utilization through the application agent since the app is aware of those metrics. However, because of the unique infrastructure of the Heroku hosting platform, it is not currently possible to collect any further server metrics other than memory and load.
Please consult these docs for further info:
NR Instances Tab: http://blog.newrelic.com/2013/03/07/new-relics-instances-tab/
Heroku Runtime Metrics: https://devcenter.heroku.com/articles/log-runtime-metrics