Send system metrics to Union Station.
Log Passenger version in the system metrics XML
'passenger-config system-metrics --xml' should end the output with a newline
Add stdin control mode to `passenger-config system-metrics`.

With this feature, the program only updates and prints metrics
every time a newline is received on stdin.
Have SystemMetricsCollector support empty newlines in /proc/stat
Move SystemMetricsCollector's swap in/out rate to from the general category to the memory category