Add support for New Relic integration

[JulienDefrance]

Currently, Racecar supports Datadog monitoring integration.

On the same pattern as datadog, what would it take to support similar functionality with their competitor NewRelic? And possibly other monitoring solutions that one might have as their APM set up within Rails?

newrelic_enabled – Whether NewRelic monitoring is enabled (defaults to false).
newrelic_host – The host running the NewRelic agent.
newrelic_port – The port of the NewRelic agent.
newrelic_namespace – The namespace to use for NewRelic metrics.
newrelic_tags – Tags that should always be set on NewRelic metrics.