Add ingress link-down discard counter to interface and subinterface counters

[ostromart]

This change introduces a new operational state counter in-link-down-discards under interface counters (/interfaces/interface/state/counters/in-link-down-discards and /interfaces/interface/subinterfaces/subinterface/state/counters/in-link-down-discards) to track ingress packets dropped due to egress link-down forwarding conditions.
The version has been bumped to 3.10.0 in openconfig-interfaces.yang.

Change-Id: I38c19a483b18e47f583c81a0b98597888405892f

Change Scope

• Description: This PR introduces in-link-down-discards to interface-common-counters-state in openconfig-interfaces.yang to count ingress packets discarded because the resolved egress interface or next-hop link selected by forwarding or routing was down.
• Backwards Compatibility: This change is fully backward compatible as it only adds a new read-only operational state leaf counter to the existing interface-common-counters-state grouping.

Use case

This counter is designed for production network debugging and automated failure pinpointing. Silent packet discards in large-scale fabrics are difficult to diagnose using generic discard counters alone. Providing an explicit counter for inbound packet drops due to egress link-down forwarding failures (in-link-down-discards) allows network operators and SRE tools to distinguish forwarding blackholes caused by downstream link failures from normal traffic discards.

Platform Implementations

• SAI specification defines an explicit debug drop reason for egress link down conditions: SAI_IN_DROP_REASON_EGRESS_LINK_DOWN (SAI Debug Counters Proposal).
• Nvidia 1 (nvswitch_interface_discards_egress_link_down for tx and nvswitch_interface_discards_ingress_tx_link_down for rx).
• Nvidia 2 (tx_discards_phy).
• Google network switches export granular ingress link-down drop telemetry via UMF and SAI debug counters.
• Broadcom / SONiC ASIC telemetry pipelines support per-port ingress drop counting for egress link-down events.

Tree View

module: openconfig-interfaces

  +--rw interfaces
     +--rw interface* [name]
        +--ro state
        |  +--ro counters
        |     +--ro in-errors?               oc-yang:counter64
        |     +--ro in-discards?             oc-yang:counter64
+       |     +--ro in-link-down-discards?    oc-yang:counter64
        |     +--ro out-octets?              oc-yang:counter64
        +--rw subinterfaces
           +--rw subinterface* [index]
              +--ro state
                 +--ro counters
                    +--ro in-errors?             oc-yang:counter64
                    +--ro in-discards?           oc-yang:counter64
+                   +--ro in-link-down-discards?  oc-yang:counter64
                    +--ro out-octets?            oc-yang:counter64

[gemini-code-assist]

Code Review

This pull request updates the openconfig-if-ethernet.yang model to version 2.19.0 and introduces a new ingress counter, in-link-down-discards, to track packets dropped due to a down egress interface or next-hop link. The reviewer pointed out that this counter represents a generic Layer 3 forwarding plane discard rather than an Ethernet-specific issue, and suggested moving it to openconfig-interfaces.yang so it can be reused across all interface types.

[navaneethyv]

Please handle the Gemini review comments before it arrives for discussion.

[ostromart]

Ack, will rework this, unfortunately there was not enough time to do so before today's meeting so we can present it next week.

[ostromart]

This PR is intended to merge after #1527 and the version reflects that.