Accumulated Metric in NHC attribute

RFC7311 describes mechanism for carrying accumulated IGP cost across BGP domains however it limits to IGP-metric only. There is a need to accumulate and propagate different types of metrics as it will aid in intent-based end-to-end path across BGP domains. This document defines BGP extensions for generic metric sub-types that enable different types of metrics to be accumulated and carried in BGP. This is applicable when multiple domains exchange BGP routing information.