Add gnsi.accounting is_truncated flags? about gnsi

[morrowc]

Seems like a good addition.
would we expect that a truncated message would instead be split over multiple messages?

[haussli]

The particular case that brought this to my attention is a subsystem that lacks a buffer large enough to hold a large accounting message (>8k IIRC). Purely an implementation limitation, but truncation is how it handled the issue. That was seen when the NMS pushed a large initial configuration to the device. This suggestion is to simply indicate that it occurred, whether it is by the originating system or some proxy service or even the destination service.

MR for that here #106

WRT splitting messages, I have not found any suggestion in the docs about how that should be done nor why GrpcService.payloads is a "repeated" field. There is no guidance about how payloads should be populated. Should the payload be split somehow? If it is split, how does a NMS correlate and sequence multiple messages? We discussed about 2 weeks ago in chat; I do not think we arrived at a conclusion. :) Should we attempt to define these processes/behaviors separately?