Fix: Use isis.metric in configuration templates

[ipspace]

After #3811, we can remove the fallback to isis.cost from the IS-IS configuration templates, resulting in cleaner templates ;)

Closes #3662

[Copilot]

🟢 Approval recommended

The templates consistently switch to isis.metric only, and the repository’s IS-IS transformation code normalizes isis.cost into isis.metric (and removes cost) before template rendering.

Pull request overview

This PR cleans up IS-IS (and SRv6-over-IS-IS) configuration templates to rely solely on the canonical transformed attribute isis.metric, removing the legacy isis.cost fallback logic now that normalization is handled during transformation (per #3811 / #3662).

Changes:

• Removed metric|default(cost) fallback logic and isis.cost guards from multiple IS-IS templates.
• Updated ArcOS IS-IS metric template comments and VRF/global parity logic to reference only l.isis.metric.
• Simplified SRv6 SR OS template metric handling to emit only srv6.isis.metric.

File summaries

File	Description
netsim/ansible/templates/vrf/arcos.j2	Uses only l.isis.metric for per-interface IS-IS metric in VRF config.
netsim/ansible/templates/srv6/sros.j2	Removes srv6.isis.cost fallback and emits only srv6.isis.metric.
netsim/ansible/templates/isis/vyos.j2	Removes l.isis.cost fallback/guards; emits metric only when l.isis.metric is present.
netsim/ansible/templates/isis/sros.openconfig.j2	Removes l.isis.cost fallback/guards in OpenConfig-rendered IS-IS interface metrics.
netsim/ansible/templates/isis/sros.j2	Removes l.isis.cost fallback/guards for SR OS IS-IS level metrics.
netsim/ansible/templates/isis/srlinux.macro.j2	Removes l.isis.cost fallback/guards in SR Linux IS-IS per-interface level metric emission.
netsim/ansible/templates/isis/nxos.j2	Removes l.isis.cost fallback/guards for NX-OS IS-IS (IPv4/IPv6) metric commands.
netsim/ansible/templates/isis/junos.isis.j2	Removes l.isis.cost fallback/guards for Junos IS-IS level metrics (incl. IPv6-unicast-metric).
netsim/ansible/templates/isis/iosxr.j2	Removes l.isis.cost fallback/guards for IOS XR per-AF interface metric.
netsim/ansible/templates/isis/ios.j2	Removes l.isis.cost fallback/guards for IOS per-interface IS-IS metrics.
netsim/ansible/templates/isis/frr.macro.j2	Removes l.isis.cost fallback/guards for FRR per-interface IS-IS metric.
netsim/ansible/templates/isis/eos.macro.j2	Removes l.isis.cost fallback/guards for EOS per-interface IS-IS metric (incl. IPv6 metric).
netsim/ansible/templates/isis/asa.j2	Removes l.isis.cost fallback/guards for ASA per-interface IS-IS metric.
netsim/ansible/templates/isis/arcos.j2	Uses only l.isis.metric for per-interface IS-IS metric and updates related comment accordingly.

Review details

• Files reviewed: 14/14 changed files
• Comments generated: 0
• Review effort level: Lite

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