Add system time to diagnostics

[peci1]

It'd be nice to also add system time to the diagnostics message. I know it is written in the message header, but having it also as a diagnostic value would allow aggregators to recognize Jetsons with unsynced clock (which happens because of the missing RTC module). We run Jetson as a slave computer publishing some topics to a master running on a different computer, and unsynced clocks caused many problems...

[rbonghi]

Hi @peci1 ,

It's a nice idea, I will implement soon :-)