Add custom instrumentation

Use a custom New Relic agent to collect and report performance metrics.

Right now we measure cache hit rates and hit/miss latency. We can add more metrics to the agent for measuring the pieces of the render pipeline and its various paths.

We need a `NAME` environment variable (as mentioned in `sample.env`) to properly differentiate between different deployments (for example, local development vs. Heroku dev service).

/cc @hampelm