add serial back to json output

[lukastribus]

Hello @job

in 38d058e some metadata outputs (Generated, Valid and Serial) were removed from rtrdump JSON output.

I think rtrdump should retain this functionality for machine based troubleshooting and monitoring purposes. The RTR serial number can otherwise only be extracted by running rtrdump in debug mode.

My use-case is that I'm monitoring serial increase over time with rtrdump and alert when there is a hung rtr server: https://github.com/lukastribus/rtrcheck/blob/d072488ab9e0f9bf9afe16f62dcd5c07a311ac1e/rtrcheck#L101

Tracked in lukastribus/rtrcheck#1

I realize there is rtrmon but it's use case seem to be quite different:

• checkrtr is a shell script that can be dropped into any monitoring systems compatible with nagios plugins and will provide monitoring for availability and hung/stuck rtr server. It requires the user to know how to access the RTR endpoint and a sufficiently large check interval, but doesn't require any other inputs from the user.

• rtrmon collects data from RTR endpoints, compares the data and provides the data for external consumption. It does not make it's own decision about whether a situation is good or bad, that is out of scope.

I don't see a lot of drawbacks to keep the rtr serial in the metadata json output of rtrdump.

Thank you,
Lukas