RT#94294: experiment with multi-metric packets

Ville Mattila sent a patch to make gauge() send multi-metric gauges.
I improved that patch and added test cases for same-metric multi value,
and different metric multi-value.

Still trying to understand if this is the correct and wanted
behaviour. Stay tuned here:

https://rt.cpan.org/Ticket/Display.html?id=94294
Mark as development (multi-metric values) version