Default stats for memory and CPU usage

[jsha]

Node provides built-in ways to get CPU and memory usage information from the process package:

https://nodejs.org/api/process.html#process_process_cpuusage_previousvalue
https://nodejs.org/api/process.html#process_process_memoryusage

It would be useful to provide these as exported variables by default. This would be in keeping with practice of other Prometheus clients, like the Go client. Would you be open to a patch?

[siimon]

There is already a PR open for this, see #25!

I think it's pretty close a merge so should hopefully be published next week.