net: add route-metric support in NetworkManager renderer

[Harshadev-24]

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Proposed Commit Message

fix(net): add route-metric support in NetworkManager renderer

This implements the `route-metric` configuration for the NetworkManager 
renderer when a `metric` value is provided in the subnet data. Prior to 
this, the renderer was silently ignoring the route metrics provided 
by the cloud data sources.

Fixes GH-6854