RTA_PRIORITY / RTA_METRICS documentation ?

Good day -
If anyone could please point me to some documentation on what
RTA_PRIORITY and RTA_METRICS route attributes actually do in
the modern Linux kernel, precisely, I'd be much obliged .
I understand RTA_METRICS is meant to represent
'administrative distance' , and higher values are greater distance and therefore
have lower precedence ?  But what about RTA_PRIORITY ?
Is numerically  lesser RTA_PRIORITY higher precedence or lower ?
Does it override or is it overridden by RTA_METRICS?  I can't seem to
find this documented anywhere - I'll have to dig it out of the kernel
source code .
I agree with the rtnetlink(7) manual page where it states 'Fill these
values in!' below the scant documentation of the RTA_* attributes .
Is there any attempt to do so out there online or on the cards?
Thanks & Regards,
Jason