Add custom instrumentation for newrelic perf/health monitoring

[humphd]

I was talking to @jbuck and he was showing me how newrelic lets you add custom tracing and instrumentation for internal bits of your app, see https://docs.newrelic.com/docs/agents/nodejs-agent/supported-features/nodejs-agent-api#custom-instrumentation-api.

This would be great for our sync steps, so that we could see how long syncs where taking, or other parts of the algorithm. There's a bunch of sample code in the Events Service:

• https://github.com/mozilla/webmaker-events-service/blob/c5974964b5c03d05991b75db916ae84aac3bbd48/server.js
• https://github.com/mozilla/webmaker-events-service/blob/e01718ab2bb13132a6ea000a199203754dbe3cf2/routes/event-controller.js#L376

We should consider which parts of our app our things that need/want to be monitored in production, and add some instrumentation.