Instrument nodes.

Results in metrics like this:

```
source=eric-macbook-air.local.rabbit@orange-hyena-01 measure#rabbitmq.os_pid=8502
measure#rabbitmq.fd_used=111 measure#rabbitmq.fd_total=16384 measure#rabbitmq.sockets_used=79
measure#rabbitmq.sockets_total=14653 measure#rabbitmq.mem_used=89014440 
measure#rabbitmq.mem_limit=247536025 measure#rabbitmq.disk_free_limit=50000000 
measure#rabbitmq.disk_free=19280224256 measure#rabbitmq.proc_used=542 
measure#rabbitmq.proc_total=1048576 measure#rabbitmq.uptime=6050337205 
measure#rabbitmq.run_queue=0 measure#rabbitmq.processors=1
```