Instrument CPU and memory usage

[watson]

Collect the following metrics:

• CPU usage (user + system)
• Memory usage (allocated heap + heap in use + rss + external)

There's a lot of other interesting metrics we could collect, but to get the ball rolling this is a nice start that will still give a lot of value.

Note that the APM Server currently does not support this. For now this is just a tracking issue. Once we start the work we'll update this issue.

[alvarolobato]

This will be implemented in the general Metrics issue we are working on soon: #422